admitted that external similarity may arise by different 

 means and in various ways. These relations of similarity 

 are of such frequent recurrence because of the limited num- 

 ber of directions of development in which changes or color 

 and design in butterflies may tend. Eimer finds the reason 

 of this small number of directions, in which development 

 may proceed, in the fact "that the elementary external in- 

 fluences of climate and nourishment on the constitution of 

 the organism are everywhere the cause of the transforma- 

 tions." fVO 



Another important point is the difference of sex. If 

 the butterflies are of different sex, the males as a rule ex- 

 liibit a more developed stage of design and color than the 

 females. These frequently present on the upper side the 

 stage of coloration, which the males present on the lower 

 side, while the upper side of the males is one stage in ad- 

 vance. It is of special significance that the characters of the 

 more advanced sex frequently correspond to those of a re- 

 lated, superior species, and occasionally to those of widely 

 separated species. Eimer endeavors to explain male pre- 

 dominance "by a more delicate and more developed, i. e., 

 more complex, chemico-physical organization of the male 

 organism." Even this development tends toward simplifi- , 

 cation, the origin of dull-black colors. Qtti(--aj£. ^/S^^elS^i^" 



This most interesting question brings Eimer into con- 

 flict with another Darwinian principle, the so-called prin- 

 ciple of "sexual election," according to which the more 

 striking characteristics of the male sexbecome strengthened 

 for the reason that females invariably give the prefer- 



80 



