FIFTY YEARS' PROGRESS IN SCIENCE. 447 



had asked no one would have answered. Now we see at a glance that the stripes 

 of the tiger have reference to its life among jungle-grasses; the. lion is sandy, like 

 the desert; while the markings of the leopard resemble spots of sunshine glancing 

 through the leaves. Again, Wallace in his charming essays on natural selection 

 has shown how the same philosophy may be applied even to birds' nests — how, 

 for, instance, open nests have led to the dull color 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 caterpil- 

 lars are full of interesting lessons; while, in other cases, specially among butter- 

 flies. 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 Von 

 Baer, the discoverer of the mammalian ovum, that we owe the great generaliza- 

 tion that the development of the egg is in the main a progress from the general 

 to the special, that zoological affinity is the expression of similarity of develop, 

 ment, and that the different great types of animal structure are the result of dif- 

 ferent 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 discovery of the Pliohippus, Protohippus, Miohippus and 

 Mesohippus, leading down from the Eohippus of the early Tertiary strata. In 

 the same way Gaudry has called attention to the fact that just as the individual 

 stag gradually acquires more and more complex antlers : having at first only a 

 single prong, in the next year two points, in the following three, and so on; so 

 the genus, as a whole, in Middle Miocene times, had two pronged horns; in the 

 Upper Miocene, three ; and that it is not till the Upper Pliocene that we find any 

 species with the magnificent antlers of our modern deer. It seems to be now 

 generally admitted that birds have come down to us through the Dinosaurians, 

 and, as Huxley has shown, the profound break once supposed to exist between 

 birds and reptiles has been bridged over by the discovery of reptilian birds and 

 bird-like reptiles; so that, in fact, birds are modified reptiles. Again, the re- 

 markable genus Peripatus, so well studied by Moseley, tends to connect the an- 

 nulose and articulate types. Again, the structural resemblances between Am- 

 phioxus and the Ascidians had been pointed out by Goodsir ; and Kowalevsky in 

 1866 showed that these were not mere analogies, but indicated a real affinity. 

 These observations, in the words of Allen Thompson, " have produced a change 

 little short of revolutionary in embryological and zoological views, leading as 

 they do to the support of the hypothesis that the Ascidian is an earlier stage in 

 the phylogenetic history of the mammal and other vertebrates." 



The larval forms which occur in so many groups, and of which the Insects 

 afford us the most familiar examples, are, in the words of Quatrefages, embryos, 



