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 
