176 Evolution and Adaptation 
“ Hence in these classes or subkingdoms, such as the Proto- 
zoa, Coelenterata, 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, antennz, 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 
