EMBRYOLOGY. 9 



nests how, for instance, open nests have led to the 

 dull colour of hen birds ; the only British exception 

 being the kingfisher, which, as we know, nests in river- 

 banks. Lower still, among insects, Weismann has 

 taught us that even the markings of caterpillars are full 

 of interesting lessons ; while, in other cases, specially 

 among butterflies, Bates has made known to us the 

 curious phenomena of mimicry. 



The science of embryology may almost be said to 

 have been created in the last half- century. Fifty 

 years ago it was a very general opinion that animals 

 which are unlike when mature, were dissimilar from the 

 beginning. It is to Yon Baer, the discoverer of the 

 mammalian ovum, that we owe the great generalisation 

 that the development of the egg is in the main a pro- 

 gress from the general to the special, that zoological 

 affinity is the expression of similarity of development, 

 and that the different great types of animal structure 

 are the result of different modes of development in 

 fact, that embryology is the key to the laws of animal 

 development. 



Thus the young of existing species resemble in many 

 cases the mature forms which flourished in ancient 

 times. Huxley has traced up the genealogy of the 

 horse to the Miocene Anchitherium, and his views 

 have since been remarkably confirmed by Marsh's dis- 

 covery of the Pliohippus, Protohippus, Miohippus, and 

 Mesohippus, leading down from the Eohippus of the 

 early tertiary strata. In the same way Boyd-Dawkins 

 and Gaudry have called attention to the fact that just as 

 the individual stag gradually acquires more and more com- 

 plex antlers : having at first only a single prong, in the 



