IV. 



Biological Theories. 



III.— THE RECAPITULATION THEORY. 



DIRECT observation has shown that, when an animal species 

 varies (i.e., becomes unhke 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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