54 THE GENETIC AND THE OPERATIVE EVIDENCE 



shown from the breeding data that it is possible to frame an explanation 

 of such a sort that the aberrant female differs from the female resem- 

 bling the male in only a single genetic factor one not sex-linked 

 (i. e., not carried by an X chromosome), but autosomal. The gene 

 would be of such a sort that it affects the female only producing no 

 visible effect on the male. Such a conclusion, if established, helps, 

 theoretically at least, toward simplifying the situation in other species, 

 for it shows that genetic factors occur whose influence is on one sex 

 alone; hence the difference between the male and one type of female 

 does in such cases result from a single gene present in both but causing 

 them to be differently colored. There would be no need, then, to assume 

 that the difference had been slowly built up by selection, but rather 

 that the difference arose at some time by a single mutant step. The 

 incorporation of the step in the species would then follow if the effect 

 of the gene were useful in mating or if it had some other primary 

 significance for the welfare of the species, the different effect produced 

 on the male and female being only an unimportant by-product of its 

 action. On the other hand, it should be emphasized that because a 

 single factor difference between the two kinds of females will explain the 

 genetic results, it does not necessarily follow that the difference did 

 arise as a single mutation. The foregoing argument does no more 

 than imply that the difference in question may have arisen in this way, 

 and if so, that the situation, as it exists, would be the more easily 

 comprehended. 



In insects and spiders, where dimorphism is as marked as in birds, 

 the mating habits have been studied by a number of naturalists. Here 

 also there are numerous accounts of the display of the male during 

 courtship. The account given by Dr. and Mrs. Peckham are particu- 

 larly detailed and call for careful consideration on account of their 

 well-recognized accuracy in observational work. Moreover, as a 

 result of their observations, along with those of Montgomery, Petrunke- 

 witsch, and others, we have really fuller information concerning the 

 courtship of spiders than of birds and of mammals. 



In the great majority of species where the sexes are different the male 

 is more brightly colored or more ornamental. For example, in a group 

 such as the Attidse of France, where both sexes are known, the Peck- 

 hams state that in 26 cases the male is more conspicuous than the 

 female ; in 55 cases the sexes are alike, or if they differ the male is more 

 conspicuous. It appears that in other genera there are cases where the 

 female is more conspicuous than the male. The Peckhams state that 

 possibly as many as 250 species are in this condition. Those females 

 with brighter colors than the males are usually well armed by strong 

 spines. When very young they are like the males and begin to assume 

 the adult form and color when they are a quarter to a third grown. 

 Whether the change depends on changes in the ovary is not known. 



