396 



EVOLUTION AND GENETICS 



bears out Darwin's answer to the objection. We may never be 

 able to explain the exact steps in the development of these organs, 

 but the belief that it has been gradual offers no greater difficulty 

 than the belief that such elaborate structures have sprung suddenly 

 into being. 



Instincts. Instincts are difficult to explain, especially when 

 they involve but a single action during the lifetime of the indi- 

 vidual. Many larval insects produce cocoons as an exercise of 

 just such instinct. Darwin answered this ol^jection by pointing 

 out that inherent mental traits vary just as do structural charac- 

 ters, and that 

 useful variations 

 might be selected 

 as readily in the 

 one category as 

 in the other. The 

 selection of a def- 

 inite mental 

 tendency would 

 result in its be- 

 coming a part of 

 the inheritance 

 of the species. It 

 would then be 

 generally ex- 

 pressed by the 

 individuals of the 

 species, whether 

 only once or of- 

 ten within a lifetime. In support of this view Darwin mentioned 

 the occurrence of peculiar instinctive actions in related species 

 in widely separated regions; "... the Hornbills of Africa and 

 India have the same extraordinary instinct of plastering up and 

 imprisoning the female in a hole in a tree, with only a small 

 hole left through which the males feed them and their 

 young when hatched." The male wrens both of Europe 

 and America build several nests, only one of which is occupied 

 by the female. These facts point strongly to similar proc- 

 esses of development of the mental and physical traits of 

 organisms. 



Fig. 206. — Median section through a primitive insect 

 eye (OrchescUa rvfcscens var. pallida), x 7,50. c, cuti- 

 cula; hy, hypodermis; pzk, pigment cells; sz, visual 

 cells; C, caudal ; R, cephalic. (From Schroder's Hand- 

 buch der Etiloinologie, after Hesse, with the permis- 

 sion of the publishing house of Gustav Fischer, Jena.) 



