234 NATURAL SCIENCE [October 



been largely determined by the direct influence of stimulation on 

 growth. 



In the reindeer, and in the bovine animals, the horns are 

 developed in both sexes, and appear at an earlier age ; those in 

 the reindeer, however, are also periodically shed, and consist of 

 bare bone, while those of the Bovidae are permanent and are 

 encased in cornified skin. I do not know whether the young 

 males and the females of the reindeer fight, or whether there is 

 any other special habit in them to explain the development of their 

 horns, but in the bovine animals it might be suggested that the 

 same stimulus of butting is applied less violently and not with the 

 same regular periodicity, and therefore has led to more permanent 

 growth. 



A brief consideration of the third kind of structural differences 

 is now to be undertaken, namely, differences in the structure of the 

 same individual at different periods of life. This is, in some respects, 

 the most important of the three kinds I have defined, for it is in- 

 separably connected with the other two. We cannot investigate the 

 origin and cause of the differences between kinships, or between 

 members of the same species, without studying the transformations 

 of the individual, for these differences arise as alterations in the 

 development of the individual. 



Now the embryos of the higher vertebrates all exhibit certain 

 characters in common, in the presence of gill-arches and gill-slits, 

 and in the origin of the limbs as bud-like outgrowths. The great 

 embryologist of the beginning of this century, Von Baer, whose 

 studies were directed principally to the higher vertebrates, formul- 

 ated the generalisation that animals of different classes resembled 

 each other closely in the earlier stages of their development, and 

 diverged more and more as they progressed toward their final form. 

 This remains true of the higher classes of vertebrates, — reptiles, birds, 

 and mammals. When the doctrine of evolution became paramount, 

 and it was seen that the comparative anatomy of the higher verte- 

 brates obviously pointed to their common derivation from ancestors 

 which were essentially fishes, the resemblance and the structure of 

 the embryos were attributed to the retention in these embryos of the 

 essential characters of the fish. The generalisation of Von Baer was 

 therefore changed into another, to wit, that in development the in- 

 dividual passed successively through the stages of its ancestors to 

 arrive at its present final condition. Haeckel gave great publicity 

 to this doctrine, calling it the biogenetic law, and formulating it in 

 the terms that ontogeny, or the development of the individual; is a 

 repetition of phylogeny, or the evolution of the race. The late 

 Professor Milnes Marshall still further popularised and established 

 the principle by embodying it in another phrase, namely, that the 



