58 THE RATE OF GROWTH [ch. 



structure between the embryo and the adult, that we are tempted 

 to look at this dift'erence as in some necessary manner contingent 

 on growth. But there is no reason why, for instance, the wing of 

 a hat, or the Jin of a porfoise, should not have been sketched out with 

 all their parts in proper proportion, as soon as any part became 

 visible.'' After pointing out with his habitual care various 

 exceptions, Darwin proceeds to lay down two general principles, 

 viz. "that shght variations generally appear at a not very early 

 period of hfe," and secondly, that "at whatever age a variation 

 first appears in the parent, it tends to reappear at a corresponding 

 age in the offspring." He then argues that it is with nature as 

 with the fancier, who does not care what his pigeons look hke 

 in the embryo, so long as the full-grown bird possesses the desired 

 qualities ; and that the process of selection takes place when 

 the birds or other animals are nearly 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 in 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 the 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 hmbs 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 manner contingent on 

 growth"; and that he should see no reason why complicated 

 structures of the adult " should not have been sketched out 



