DEGENERATION IN BREEDING INSTINCTS. 75 



produced by the Helix are different from those produced by the Acha- 

 tinella. The short-legged lamb, from which sprang the Ancon sheep, 

 was not the suckling of a lioness, but had a sheep for its dam. In 

 other words, the segregations and survivals of one generation control 

 the types of heredity and variation (including mutation) in the next 

 generation. 



Second Lesson. If the selection of individual variations is necessary 

 for the maintenance of the normal standard of eyesight in the human 

 species, is it not possible that the same necessity exists in other 

 species? And may it not be true that many other inherited endow- 

 ments are subject to gradual decay when the standard of selection is 

 lowered? If we find, in a given country, that the mothers who have 

 to feed their babes on artificial substitutes for mother's milk lose a 

 larger per cent of their children than do those who are able to give 

 suck, does it necessarily follow that the power of giving suck is in- 

 creasing from generation to generation among the people of that 

 country? In the language of the statistical method, is it not possible 

 that the "skewness" of the "frequency curve" (for different grades 

 in the power of furnishing milk) might, in such a case, give some 

 indication of the selection that is taking place, and that, at the same 

 time, the statistics of successive generations might show that there 

 was no gain in the power? Or, in such a case, would there be no 

 skewness in the frequency curve, though there is constant selection 

 that results in the maintenance of a constant standard ? 



20. Degeneration in Breeding Instincts. 



In the case of the Old World cuckoo it may be a question whether 

 the loss of the maternal instinct (or rather of this series of instincts) , 

 came in a single generation, by one mutation, maintaining its type 

 with constancy from the first; or by several successive mutations, 

 each mutation being added to the previous ones, and being persist- 

 ently inherited; or whether the process has been a very gradual ac- 

 cumulation of individual tendencies through the success of aberrant 

 individuals in leaving descent, and so lowering the general standard 

 of service for the whole species. There are, however, certain facts 

 that point toward the last of these as the process by which the degen- 

 eration has taken place. F. M. Chapman, in his Handbook of Birds 

 of Eastern North America, notes that ' ' Many species [of cuckoo] are 

 remarkable for the irregularity of their breeding habits." Of the 

 Ani, a genus of the same family, he says : "The Anis are communistic, 

 and build but one nest, in which several females lay and share the 

 task of incubation." Now, it is manifest that, in a community of this 



