22 



EVOLUTION AND GENETICS 



tionship may be in the form of constant association, as in the many- 

 colonial animals (Fig. 4) and the social forms (Fig. 5), or it may be 

 in a succession such as the alternation of different reproductive 

 types, and the succession of stages in metamorphosis (Fig. 6). 

 In all of these cases, the ability of the various forms to produce each 

 other is abundant evidence of specific unity, but mere lack of in- 

 formation has often led to the sepa- 

 ration of subspecific forms. 



Sexual Differences. Sex is a com- 

 mon and sometimes a conspicuous 

 example of intraspecific difference 

 (Fig. 7) . In addition to those differ- 

 ences which arc essential to comple- 

 mentary reproductive functions, 

 others of an apparently unrelated 

 character often appear. These are 

 called secondary sexual characters. 

 The long tail feathers of male 

 turkeys, chickens, peacocks and 

 pheasants, bright colors in the male 

 sex of many species of birds, the 

 mane of the lion, and other charac- 

 ters are well known examples. 

 Sensory organs in many male insects 

 and the horns of bucks and rams 

 are more evidently useful to the 

 animals. Even in the human race 

 the sexes differ fundamentally, for 

 accumulations of subcutaneous 

 adipose tissue give the body of the female characteristic round- 

 ness of outHne, while the growth of whiskers and of the vocal 

 cords brings about equally characteristic male development. 

 Among the invertebrates sexual dimorphism sometimes involves 

 the general structure and appearance, as in the common prome- 

 thea moth, in which color, pattern and shape of the wings differ, 

 as well as the form of the antennae and development of sense 

 organs. The sexes of some of the parasitic worms are even more 

 diverse. While this is true of both flat- and roundworms it is 

 probably nowhere more conspicuous than in Schistosoma haemato- 

 bium, a blood fluke of the eastern Mediterranean region (Fig. 8). 



Fig. 7. — Enlheus pdcua Linn. 

 A, male; B, female. The male 

 is dark brown with an orange- 

 red band and transparent 

 orange-yellow spots. The 

 female is brown with white 

 markings. 



