A HALF-CENTURY OF SCIENCE. 69 



ated 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 mam- 

 malian ovum, that we owe the great generalization that the develop- 

 ment of the egg is in the main a progress from the general to thc- 

 special; in fact, that embryology is the key to the laws of animal devel- 

 opment. 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. In the 

 same way Gaudry has called attention to the fact that, just as the indi- 

 vidual stag gradually acquires more and more complex antlers hav- 

 ing 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 Mio- 

 cene 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 remarkable genus Peripatus, so well studied by 

 Moseley, tends to connect the annulose and articulate types. 



Again, the structural resemblances between Amphioxus and the 

 Ascidians had been pointed out by Goodsir ; and Kowalevsky in 1866 

 showed that these were not mere analogies, but indicated a real affin- 

 ity. These observations, in the words of Allen Thomson, " have pro- 

 duced a change little short of revolutionary in embryological and zoo- 

 logical views, leading as they do to the support of the hypothesis that 

 the Ascidian is an earlier stage in the phylogenetic history of the mam- 

 mal 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 Qua- 

 trefages, embryos, which lead an independent life. In such cases as 

 these, external conditions act upon the larva? as they do upon the ma- 

 ture form ; hence we have two classes of changes, adaptational or 

 adaptive, and developmental. These and many other facts must be 

 taken into consideration ; nevertheless, naturalists are now generally 

 agreed that embryological characters are of high value as guides in 

 classification, and it may, I think, be regarded as well established that, 

 just as the contents and sequence of rocks teach us the past history of 

 the earth, so is the gradual development of the species indicated by 

 the structure of the embryo and its developmental changes. When 

 the supporters of Darwin are told that his theory is incredible, they 

 may fairly ask why it is impossible that a species in the course of hun- 

 dreds of thousands of years should have passed through changes which 

 occupy only a few days or weeks in the life-history of each individual. 



