176 Evolution and Adaptation 



" Hence in these classes or subkingdoms, such as the Proto- 

 zoa, Ccelenterata, Echinodermata, Scolecida, secondary sexual 

 characters, of the kind which we have to consider, do not 

 occur ; and this fact agrees with the belief that such charac- 

 ters in the higher classes have been acquired through sexual 

 selection, which depends on the will, desire, and choice of 

 either sex." 



There are some cases, however, in which animals low in 

 the scale show a difference in the ornamentation of the two 

 sexes. A few cases have been recorded in the roundworms, 

 where different shades of the same tint distinguish the sexes. 

 In the annelids the sexes are sometimes so different, that, as 

 Darwin remarks, they have been placed in different genera 

 and even families, "*yet the differences do not seem to be of 

 the kind which can be safely attributed to sexual selection." 

 In regard to the nemertian worms, although they "vie in 

 variety and beauty of coloring with any other group in the 

 invertebrate series," yet Mcintosh states that he "cannot 

 discover that these colors are of any service." In the cope- 

 pods, belonging to the group of lower Crustacea, Darwin 

 excludes those cases in which the males alone " are furnished 

 with perfect swimming legs, antennae, and sense organs ; the 

 females being destitute of these organs, with their bodies 

 often consisting of a mere distorted mass," because these 

 extraordinary differences between the two sexes are no doubt 

 related to their widely different habits of life. Nevertheless, 

 it is important to observe that such extreme differences may 

 exist in cases where sexual selection cannot come in, because 

 of the absence of eyes in the female. 



In regard to another copepod, Saphirina, Darwin points 

 out that the males are furnished with minute scales, which 

 exhibit beautiful changing colors, and these are absent in 

 the females ; yet he states that it would be extremely rash 

 to conclude that these curious organs serve to attract 

 the females. Differences in the sexes are also found in one 



