The similarity between the artificial selection of the breeder and the natural selection by means of which Darwin first explained evolution has often been pointed out. By repeated citation it has become very trite, but, like most classic examples, it is a very useful one. We can cover much important preliminary ground by briefly considering this Darwinian theory.
In his "Origin of Species" Darwin brought forward a wealth of evidence to prove that like tends to produce like (that two Beagles will have Beagle puppies) that the offspring always differ from their parents (that no two Beagles were ever identically alike). These are the two fundamental laws of heredity and variation. Darwin said in substance that, granting the truth of these, the fierce struggle for existence always going on in nature would explain the differences in the various species through the action of natural selection, or the survival of the fittest.
In order to get this plainly before us, let us take an imaginary case and reduce it to its very simplest form. Let us suppose that by the invasions of some stronger animal a remote ancestor of the dog is driven out of a wooded, mountainous country and forced to live on the bare, open plains. Let us leave out of consideration every factor except the procuring of his food. On the plains he has no cover to aid him in stalking his prey, and he cannot, therefore, approach his victim so closely as before.
It is evident that those wild dogs which can run ever so little more swiftly will stand just so much better chance of "making a living" in the new country. Moreover, skulking through the deep shadows, a dark color would make him less conspicuous, and, whether he was hunting or being hunted, would be a distinct advantage. In the open, however, against the light shades of the rocks and dried grasses, a dark color would be a decided handicap, but a dun or a fawn would be an advantage. Obviously, any dogs that were a bit lighter in coloring would be enabled to creep just so much nearer to a feeding antelope. Any dogs swifter of foot and lighter in color will be better able to live on the plains than their slower, darker brothers and sisters. They will have just that slight advantage in getting food and escaping enemies that in the life and death fight for existence will enable them to win out. Natural selection will be working for speed and light coloring.
Those wild dogs, in our example, that are swiftest and lightest will, we have seen, be better able to live on the plains. The chances are in favor of their being the parents of the following generation. Their offspring will contain individuals some lighter and some darker than their parents, and also some swifter and some slower. On the average, however, the second generation will be lighter and swifter than the generation to which their parents belonged. Again the swiftest and the lightest will survive, and these selected individuals will almost certainly be swifter and lighter than their parents, who were in turn the swiftest and lightest of their own generation. And so on for ages.
Eventually the comparatively slow moving, dark colored wild dog that came down to the plains from the mountains has become a fast running, dun colored one. Any dog fancier knows that this will mean that a thick-set dog with erect shoulders and straight hocks has become a lithe, racy animal with sloping shoulders and well let down hocks. The demand for increased speed has forced important changes in conformation.
Remember, moreover, that we have only considered the single factor of procuring food. Not only this, but every other condition of his life has been radically changed by the migraion from the wooded hills, and each change may have a very direct effect. It is not hard to imagine that, given sufficient time, the dog might be so changed as not to be recognized as belonging to the same species.
This is briefly the explanation of evolution according to the Darwinian theory of natural selection. It is well to note that all biologists agree in their belief in evolution, and the disputes that are engaged in are not over the question of evolution, but over explanations of how evolution has been accomplished and how it is working to-day.
Fig. 1.—Typical Cells of the Frog. A, surface of the tongue;
It is plain, for example, that natural selection cannot account for all the phenomena of evolution. It can only act as a sieve, winnowing the fit from the unfit. As Arthur Harris has said, "Natural selection may explain the survival of the fittest; but it cannot explain the arrival of the fittest." Neither can it account for any useless character, like the tail of the peacock, for example, which is certainly no help to the bird in its struggle for existence. Darwin himself recognized these objections and suggested sexual selection as a supplementary force. Other theories have been advanced, but they do not interest us. The points to be brought out here are that evolution is accepted by all biologists, and natural selection is acknowledged to be one of the most important factors in evolution. The laws of heredity and variation are fundamental beliefs.
Both artificial selection and natural selection rest upon heredity and variation. They differ in that natural selection is passive and usually very slow in its action, but artificial selection, in the hands of a breeder, who is able to work backward through pedigrees and forward by judging his breeding stock not only by their own qualities but also by the qualities of their existing offspring, is more active and yields quicker results.
Fundamentally the breeder's work is selection, and all selection is based upon the workings of heredity and variation. If like did not beget like all would be chaos. What could a breeder do if a Beagle dog bred to a Beagle bitch resulted in a litter consisting of a Gordon Setter, a French Bulldog, a dog with undreamed-of characteristics, and maybe a lion cub and a young raccoon thrown in for good measure? This sounds like extravagant foolishness because the law of heredity is so universal that any exception seems beyond comprehension.
On the other hand, if the mating of two Beagles produced a litter all exactly a mean between the two parents, a perfect combination of all their physical points and mental characteristics, breeding would be absolutely fruitless. In a very short time we should have all Beagles reduced to a dead level average. Every single one would be the same size, have the same conformation and markings, and be identical in voice, speed, nose, and disposition. Thanks to variation a selection is possible, and thanks to heredity a selection is effective.
Fig. 2.—Cell division showing splitting of nucleus (N), division
Plainly, a breeder cannot know too much about these two basic laws upon which all his work depends. Before taking them up, however, we can advantageously look into the mechanical side of reproduction. It has an important bearing upon our work.
Most people know that the whole body of any animal is made up of a great colony of minute cells. Bone cells and muscle cells, for example, differ in many ways, but all cells have much in common. Every cell is filled with a gelatinous substance called protoplasm in which is a tiny speck, the nucleus. (Fig. 1.) All functions of the body are carried on by cells, each doing its own work. Moreover, all growth in a body is accomplished by means of cell division, in which the nucleus splits in half and separates, each part carrying with it half of the protoplasm. (Fig. 2.) Finally the whole colony of cells that makes up the living organism has arisen from a single cell. In this minute germ cell are hidden away all the secrets of heredity and variation, secrets that man has been trying for years to wrest from it.
Dogs, like the other higher animals, reproduce sexually. The combination of the male and female germ cells is necessary for the development of a puppy. The sperm, or male germ cell (Fig. 3, A) is even many times smaller than the microscopic female cell. It is a free moving body, shaped like a spear with a roundish head and a whip-like tail, which is used, like a propeller, for locomotion. These sperm cells are produced literally in millions. The ovum, or female cell (Fig. 3, B) is larger, round in shape, and without the power of motion. Like other cells, both the sperm and the ovum contain protoplasm and a nucleus.
Fig. 3.—Germ Cells. A, Sperms, (i) Snail, (2) Frog,(3) Bird;
Fertilization takes place when the sperm actually unites with the ovum. In some way that is not yet thoroughly understood, the ovum has the power of attracting to itself any sperms that are near it. They move toward the ovum, tails being used as propellers. Finally one—but only one—sperm pushes its spearlike head in through the wall of the ovum, the tail being left behind. The head of the sperm, containing its nucleus and protoplasm, now travels to the nucleus of the ovum. The two nuclei fuse together. Fertilization has taken place. The tiny germ cell is definitely started on its way toward the development of a new individual. It is endowed with tremendous energy and divides repeatedly after a set pattern till in the place of one cell there are many. It grows, fed by the mother to which it fastens itself, and in the course of time it develops into a puppy and is born. The details of the development of the embryo inside its dam are highly interesting, but since a breeder can have no direct influence over that development once fertilization has taken place, it would be out of place for us to follow these studies here.
Let us, however, return a moment to the sperm and the ovum. In the microscopic cells are hidden away all the possibilities of a new dog, a new dog that will be of the same variety as his parents, at the same time differing from them in a thousand ways. For years scientists have been patiently trying to wrest from these tiny specks of life their wonderful secrets of heredity and variation. Much is still a mystery, but many things have been learned that can be of great value to dog breeders.
There have been a number of explanations proposed to account for the well known fact that like tends to produce like. One of the earliest of the theories of heredity assumed that the great number of different parts that make up each adult animal each gave off little buds which were carried by the blood to the reproductive organs and stored away in the germ cells. According to this view, a miniature duplicate of the parent was tucked away in its own germ cells, each sperm and each ovum containing a complete set of buds representing every part of the body of the animal that produced it.
This theory has been discarded. One obvious objection to it is to be found in the case of mutilations. If each part of the body gives off buds to be stored away in the germ cells, it follows that should a certain part be removed in the parent it ought to be lacking in the offspring. All Spaniel puppies would therefore be born with short tails, since the buds for the last few joints at the end of the tail should be missing from both parents. Other theories have been advanced, but the one in which we are particularly interested is the germ plasm theory of the great German biologist, August Weismann.
Fig. 4.—Diagram of the Germ Plasm Theory. The Squares
In all its details this germ plasm theory is somewhat complicated to one who has not had scientific training, but it may be briefly explained. According to Weismann's conception the whole of the fertilized ovum is not used up in the development of the new individual. A part of it is put aside and remains unchanged for the special purpose of making the germ cells which this new individual will use to beget the following generation. The accompanying diagram (Fig. 4) will help to make clearer this important idea. The black dots (A1, A2, etc.) are the germ plasm that is passed along complete and intact from generation to generation. The white squares are the individuals of the different generations, each an elaborated collection of specialized cells developed for the express purpose of protecting and supporting the germ cells. This idea of heredity usually associated with Weismann's name is the most reasonable explanation of the observed phenomena that we have, and other men, notably Haeckel, Owen, Brooks, Rauber, and Galton, have been contributors to this important conception. The setting aside of certain cells at the very beginning of embryonic development and later making reproductive organs out of them has been actually seen under the microscope. There are other important facts supporting the germ plasm theory, and, as we shall see, it offers a sound explanation for most of the observed facts of heredity.
If the germ cells which give rise to the offspring are continuous with those that gave rise to the parent, and this is the generally accepted belief among biologists, breeders will find new ideas of great practical benefit presented to them. It follows that the transmission of the individual characteristics of the sire and the dam is not nearly so direct as dog breeders are prone to take for granted. "Like father like son" is not nearly so accurate a saying as "like tends to produce like." This understanding of heredity throws the emphasis off the individual and upon his or her units of inheritance. We are not mating a dog and bitch of certain physical points and peculiar characteristics. We are combining two germ plasms, each bearing certain factors of inheritance. This is a great deal more than a mere difference in terms.
The direct connection from generation to generation is through the germ plasm, and any variation in the offspring lies, not in the development or characteristics of the parents, but only in those things that may affect the nature of their germ plasm. It is as if you had a number of similar balls of soft clay, which will represent the germ plasms. You can put these clay balls into paper bags of different sizes and colors and shapes. By so doing, however, you will not in the least change the clay balls no matter how different they may look in their different bags. Moreover, it would be quite possible to put two identical balls in two like bags, and yet to change the nature of the balls markedly by putting one in a hot oven for a few hours. The two bags will still appear quite alike, but inside of one will be a baked, hard ball of brick, and in the other a damp, plastic ball of clay. According to the germ plasm theory of heredity, the germ plasm, the balls of clay in our example, are passed along from generation to generation directly, and the differences in the individuals bearing the germ plasm is not a sure test of differences in their germ plasm.
That there are things, however, that affect the nature of the germ plasm is very evident, or else there would be no variation. As a matter of fact we all appreciate that there is constant and infinite variation. The germ plasm theory must be able to reasonably explain this, or it will not be acceptable.
Naturally the combination of two different germ plasms in the mating of a dog and bitch would cause a variation. There must, however, be other factors, or all the puppies of a litter would be identical, each a mean between its two parents. This certainly does not tally with common observation. There must be other factors at work.
We have seen that the fertilized ovum is a combination of the male sperm and the female ovum, each, according to the accepted theory, carrying its own germ plasm with its individual units of heredity. As has been pointed out, all growth is by cell division in which the nucleus divides, each half taking with it half of the protoplasm. Fertilization is essentially the direct opposite of this, for two cells fuse. It is easy to see that were two cells to combine we should have, in a quantitative way, a double cell with a double nucleus and double the regulation amount of protoplasm. This piling up would continue, and in the next generation we should have a quadruple cell. Without going into the technical details, it may be said that when the sperm cells are maturing in the male reproductive organs they reach a certain stage when they divide into four parts, each containing a fourth of the original nucleus and a fourth of the protoplasm. The ovum also undergoes reduction, as this is called. In this case, however, three-fourths of the original nucleus is extruded from the cells, thrown off, and lost so far as reproduction is concerned. The result of this is that the sperm and the ovum are each, in a quantitative sense of the word, a quarter cell. When they combine and fuse they make a half cell, which amounts to the same thing as a full cell dividing.
This reduction is an actual fact and has been often seen in the germ cells of many different kinds of animals. Of course, only the main features of reduction have been outlined, and all confusing technical words have been stripped away. Some knowledge of the subject, how ever, is important to the breeder who would have a better understanding of the nature of heredity.
The reduction of the sperm and ovum is regarded as a factor that introduces a primary cause for variation. The interpretation of the observed facts is as follows. The germ cells are considered as being made up of definite and distinct units of inheritance, called by Weismann determinants. By dividing the germ cell first up and down and then crossways, the determinants are divided in equal numbers but in different combinations. A homely example would be to cut a cherry pie into quarters. Each quarter would contain approximately the same number of cherries—but they will be different cherries. So, reduction of the germ cells is supposed to yield an infinite number of different combinations of the units of inheritance, resulting in infinite variation.
The importance to a practical breeder of these modern conceptions of inheritance will be better appreciated when we come to apply them directly to the specific problems that fanciers must solve. The dog breeder should get them clear in his mind, for we shall use them later. Weismann's theory of heredity considers the ancestral germ plasm as being passed on directly from generation to generation and regards the body as a case developed round the vital germ cells for their nourishment and protection. Reduction of the sperm and the ovum is a well known fact that is interpreted as supplying a fundamental cause for variation, in so much that the reduced cells will each contain different combinations of the units of heredity, or determinants.