Ill] A PASSAGE IN DARWIN 87 



grown up — at least on the part of the breeder, and presumably in 

 nature as a general rule. The illustration of these principles is set 

 forth as follows: "Let us take a group of birds, descended from 

 some ancient form and modified through natural selection for 

 different habits. Then, from the many successive variations having 

 supervened iA the several species at a not very early age, and having 

 been inherited at a corresponding age, the young will still resemble 

 each other much more closely than do tl^e adults — just as we have 

 seen with the breeds of the pigeon. . . . Whatever influence long- 

 continued use or disuse may have had in modifying the limbs or 

 other parts of any species, this will chiefly or solely have affected 

 it when nearly mature, when it was compelled to use its full powers 

 to gain its own living ; and the effects thus produced will have been 

 transmitted to the offspring at a corresponding nearly mature age. 

 Thus the young will not be modified, or will be modified only in a 

 shght degree, through the effects of the increased use or disuse* of 

 parts." This whole argument is remarkable, in more ways than 

 we need try to deal with here; but it is especially remarkable that 

 Darwin should begin by casting doubt upon the broad fact that a 

 "difference in structure between the embryo and the adult" is 

 "in some necessary matter contingent on growth"; and that he 

 should see no reason why compUcated structures of the adult 

 "should not have been sketched out with all their parts in proper 

 proportion, as soon as any part became visible." It would seem to 

 me that even the most elementary attention to form in its relation 

 to growth would have removed most of Darwin's difficulties in regard 

 to the particular phenomena which he is considering here. For 

 these phenomena are phenomena of form, and therefore of relative 

 magnitude ; and the magnitudes in question are attained by growth, 

 proceeding with certain specific velocities, and lasting for certain 

 long periods of time. And it seems obvious accordingly that in any 

 two related individuals (whether specifically identical or not) the 

 differences between them must manifest themselves gradually, and 

 be but Httle apparent in the young. It is for the same simple 

 reason that animals which are of very different sizes when adult 

 differ less and less in size (as well as form) as we trace them back- 

 wards to their early stages. 



Though we study the visible effects of varying rates of growth 



