IV. 
Biological Theories. 
Ill—THE RECAPITULATION THEORY. 
IRECT observation has shown that, when an animal species 
varies (i.€., becomes unlike what it was before) in adult 
structure, those stages of the development which are nearest the 
adult undergo a similar but usually smaller change. This is shown 
in domestic species by the observations of Darwin, and the 
result is in exact harmony with the well-known law of Von Baer, 
which refers to natural species, both nearly related and very widely 
dissimilar. 
Von Baer’s observations as well as Darwin’s, and as well as 
those of every student who has ever compared the embryos of two 
vertebrate-species, may be summarised as follows :— 
Animals which, though related, are very unlike in the 
adult state, resemble each other more closely in early stages 
of development, often, indeed, so closely as to be indis- 
tinguishable in those early stages. As development proceeds, in such 
species, the differences between the two embryos compared become 
more and more pronounced. 
If similar comparisons could be instituted between an ancestral 
species and its much modified descendants, there is no reason for 
doubting that a similar result would be reached. This, indeed, has 
been done in the case of some breeds of pigeons, which we have 
excellent reasons for believing to be descended from Columba livia. 
True, C. livia is not a very remote ancestor, but I do not think that 
will vitiate the argument. Let me quote Darwin verbatim: ‘“ As we 
have conclusive evidence that the breeds of the Pigeon are descended 
from a single wild species, I have compared the young within twelve 
hours after being hatched ; I carefully measured the proportions (but 
will not here give the details) of the beak, width of mouth, length ot 
nostril and of eyelid, size of feet, and length of leg in the wild parent- 
species, in pouters, fantails, runts, barbs, dragons, carriers, and 
tumblers. Now some of these birds, when mature, differ in so extra- 
ordinary a manner in the length and form of beak, and in other 
characters, that they would certainly have been ranked as distinct 
genera if found in a state of nature. But when the nestling birds of 
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