RECAPITULATION 203 



different sense from that in which it is used by other 

 biologists, Bateson for example. The word has 

 been applied previously to variations which form a 

 continuous series in a large number of individuals, 

 each of which differs only slightly from those most 

 similar to it. No two individuals are exactly alike, 

 and thus such continuous variations are universal. 

 According to the theory of natural selection the 

 course of evolutionary change in any organ or 

 character would form a similar continuous series, 

 the mean of each generation differing only by a small 

 difference from that of the preceding. According 

 to the modern mutationists such small differences 

 are to be called fluctuations, and have no effect on 

 evolution at all, are not even hereditary, are not due 

 to genetic factors in the gametes. Discontinuous 

 variations, on the other hand, are as a rule differences 

 in an individual from the normal type and from its 

 parents of considerable degree, and are conspicuous : 

 these are what are called mutations. 



The mutationists and Mendelians have not shown 

 how the essential characteristics of mutations are to 

 be reconciled with the facts of metamorphosis, or 

 with recapitulation in development which is so often 

 associated with metamorphosis. T. H. Morgan is 

 the only mutationist, so far as my reading has gone, 

 who has attempted to do this, and he seems to me to 

 have failed to understand the difficulties or even the 

 nature of the problem. He points out that the 

 embryos of Birds and Mammals have gill slits re- 

 presenting the same structures as those of the adult 

 Fish, but the young stage of the Fish also possessed 

 gill slits, therefore it is ' more probable that the 

 Mammal and Bird possess this stage in their de- 



