Differentiation. 293 



might be arrested development. Others in which organs are excessively 

 developed. 



The distinction of Species was, by the old naturalists, thought to be an everlasting and 

 impassable barrier or fence separating the tribes of animals and forming the limits 

 within which the children might differ in form from their parents, and new varieties 

 thus be set up, but within which the species must forever remain, as they claimed they 

 always have remained from the day of their creation. . This theory contemplates a super- 

 natural creation of all the animal tribes in pairs, each pair the originator of a species. 

 This theory is rendered impossible in view of all the facts that have been cited showing 

 the modifying and selective agencies at work on the organism, and the modifications 

 that have been actually accomplished by them under human observation. But there is 

 still another class of facts that may be said to finish the subversion of the ancient theory 

 of species, and that is the facts of hybridism, or the mixing of species in reproduction. 

 Hundreds of such mixtures are known to have taken place between animals of various 

 species, and we are left without the slightest assurance that the blood of any species has, 

 in a state of nature, remained pure for many generations. The following are some of 

 the crosses known to have taken place either in a state of nature or in captivity to man : 

 Cynocephalus Mormon, or mandrill, 1 and Macacus Cynomolgus, or hare-lipped macaque 

 (baboon); rabbit and hare; tiger and lion; leopard and jaguar; polar bear and 

 brown bear ; masked pig of Africa and domestic Berkshire ; dama vulgaris 2 and dama 

 mesopotamica ; equus onager (wild ass) and equus hemipus; Burchells zebra and horse; 

 Burchells zebra and ass ; Burchells zebra and equus hemionus, or Dzegguetai ; polecat 

 and ferret; wild cat and domestic cat; African leopard 3 and black panther of Java; Yak 

 and common cow ; bison and common cattle; dog and jackal; dog and wolf; sheep O. 

 musimoii and O. cycloceros ; deer, C. Virginianus and C. macrotis ; goat and sheep 4 ; 

 5 deer and sheep; Bactrian two-humped camel, and Arabian one hump dromedary; 

 modern pigeon and turtle dove ; greenfinch and goldfinch. Different species of ducks 

 with each other, as the mallard with the pintail ; different species of snakes ; of trout 

 with carp ; many insects, as hawk moths with several kinds of peacock moths, &c. Of 

 the foregoing many of the hybrid products are in themselves fertile, and would almost 

 certainly thus originate new species. And, as before said, we have no assurance that 

 many species have not already arisen so. 



It is proved that hybrids are not merely copies of their parents, taking some of their 

 characteristics from one and the rest from the other, but they always, while partaking 

 of the parental characteristics, also take on others foreign to either parent. The mule 

 and hinny are almost as unlike each other as are the ass and horse. We may regard the 

 embryo as a crystalloid body made up of two sets of complemental elements, one set of 

 which is furnished by each of the parents ; and as receiving from the mother the neces- 

 sary materials for its growth, as the crystal gets the materials for its growth from the 

 solution in which it is immersed. Obviously in every cross, the crystal being the result 

 of a new combination of complemental elements, it differs from the accustomed type. 

 We have seen that different organisms will grow in the same nutritive solution or infu- 

 sion, but will become more or less modified themselves. There are two possible crosses, 

 such as the two from which the mule and the hinny result, so that the union of any two 

 tribes may produce two hybrids quite different from each other and from their parents. 

 If these hybrids are fertile, and they often are, in a few generations they might set up 

 for species with their parent stocks as varieties. 



In view of the foregoing it is no surprise to learn that naturalists have great difficulty 

 to draw close and definite lines between species. They not only grade into each other 

 until the distinctions become too slight to justify different names, but the species persist 

 in crossing back and forth over every line that is drawn; those varieties near the line 

 possessing qualities on both sides of it, related to a species on this side the line by re- 

 semblances of one kind and to a species on the other side by resemblances of another 

 kind. The Editors of Cuvier say in regard to the classification of the cat tribes " That it 

 is quite impossible to subdivide the genus Felis into definite sections, and that every 

 attempt of this kind hitherto made has consequently proved a complete failure; the 

 transition into the Lynxes is most gradual, and the spotless species, as the Lion, Puma, 

 &c., are marked like the rest when young. Those species, however, which affect the 

 open country, as the Lion and Leopard, have the pupil of the eye contracting to a,point, 

 whereas in those which inhabit forests, as the Tiger and domestic Cat, the pupil closes 



1 Semper, 353. * A rhinoceros the size of a rabbit. 



3 The word Leopard is itself a hybrid ( Leo, lion; Pardus, panther). 



4 Buffon. 5 Nott & Gliddon. 6 Semper. 



