Page:The American Cyclopædia (1879) Volume VII.djvu/21

 EVOLUTION 13 phenomena of nature. The following facts have been oifered as throwing light upon the way in which the diversities of life have ori- ginated. Organic beings differ from inorganic in their modlfiability. They are capable in various degrees of adaptation to new condi- tions. Plants taken from their native situa- tions and cultivated in gardens undergo changes so great as often to render them no longer rec- ognizable as the same plants. The muscles are strengthened by exercise and the skin thick- ened and hardened by pressure, while the bones of men who put forth great physical exertion are more massive than the bones of those who do not labor. In the words of Mr. Spencer : " There is in living organisms a margin of functional oscillations on all sides of a mean state, and a consequent margin of structural variation." These variations may become fixed through the law of he- reditary descent. It is the law of trans- mission of characters which preserves species and varieties from generation to generation, oaks being always derived from oaks and dogs from ancestral dogs. It is not only the normal qualities that are perpetuated, but malforma- tions, diseases, and individual peculiarities are also transmitted. While offspring tend to grow in the likeness of parents, they also tend to grow in unlikeness ; while moulded upon the parental type, the resemblance is usually im- perfect. Nor are variations confined to any particular organs or characters, but they may be manifested by every part, quality, or in- stinct of the creature. These divergences may be selected and fixed by breeding so as to give rise to new kinds or varieties. Nature begins the variation, art secures its perpetuation and increase. How profound are the modifications that may be thus produced is shown in the numerous breeds of dogs, all of which belong to the same species. Not only have they reached extreme diversities in size (the largest being, according to Cuvier, 100 times larger than the smallest), but in muscular, bony, and nervous development, in form, strength, fleet- ness, and variety of instinct and intelligence, their divergences are almost equally remark- able. Domestic pigeons afford another ex- ample of the great plasticity of the living or- ganism, by which it can be moulded into the extremest diversities. Naturalists believe that from a single species, the wild rock pigeon, there have arisen no fewer than 150 kinds that breed true or hold to the variety ; and how deep have become the differences among them is thus stated by Prof. Huxley: "In the first place, the back of the skull may differ a good deal, and the development of the bones of the face may vary a good deal ; the beak varies a good deal ; the shape of the lower jaw varies; the tongue varies very greatly, not only in correlation to the. length and size of the beak, but it seems also to have a kind of independent variation of its own. Then the amount of naked skin round the eyes and at the base of the beak may vary enormously ; so may the length of the eyelids, the shape of the nostrils, and the length of the neck. I have already noticed the habit of blowing out the gullet, so remarkable in the pouter, and com- paratively so in the others. There are great differences, too, in the size of the female and the male, the shape of the body, the number and width of the processes of the ribs, the development of the ribs, and the size, shape, and development of the breast bone. We may notice, too (and I mention the fact because it has been disputed by what is assumed to be high authority), the variation in number of the sacral vertebrae. The number of these varies from 11 to 14, and that without any diminution in the number of the vertebrae of the back or of the tail. Then the number and position of the tail feathers may vary enor- mously, and so may the number of the primary and secondary feathers of the wings. Again, the length of the feet and of the beak, although they have no relation to each other, yet ap- pear to go together ; that is, you have a long beak wherever you have long feet. There are differences, also, in the periods of the acquire- ment of the perfect plumage, the size and shape of the eggs, the nature of flight, and the powers of flight, so-called 'homing' birds having enor- mous flying powers ; while on the other hand, the little tumbler is so called because of its extraordinary faculty of turning head-over- heels in the air, instead of pursuing a distinct course. And lastly, the dispositions and voices of the birds may vary. Thus the case of the pigeons shows you that there is hardly a single particular, whether of instinct or habit, or bony structure, or of plumage, of either the internal economy or the external shape, in which some variation or change may not take place, which by selective breeding may become perpetuated and form the foundation of and give rise to a new race." Nor is this variation confined to domestic animals. Wild species both of plants and animals vary, become diversified, and give rise to new varieties. As many as 28 varieties of oak have been made out within the limits of a single species. The wolf species exhibits some 15 varieties, and lions, tigers, bears, hyae- nas, foxes, birds, reptiles, and fishes all exhibit marked varieties, which show that wild species undergo modification in a state of nature. What was needed to make out the analogy of variation between wild and domesticated ani- mals was to discover some process in nature which is the equivalent of human agency hi breeding. Mr. Darwin believes that he has discovered this process, and calls it the princi- ple of " natural selection." He says that living beings in a state of nature are subject to cer- tain external conditions, such as climate, situa- tion, character of soil, and exposure to enemies, by which they are surrounded and limited. They are endowed with enormous powers of increase, so that any one of the hundreds of thousands of species of plants or animals, if all