238 THE EVOLUTION THEORY 



corals, and Hydroid polyps. In some other classes the sense organs 

 are too poorly developed, and the eyes in particular too imperfect to 

 be excited in different degrees by any peculiarities in the appearance 

 or behaviour of the males. This is what Darwin meant when he 

 ascribed to these animals 'too imperfect senses and much too low 

 intelligence ' ' to estimate the beauty or other attractive points of the 

 opposite sex, or to feel anything like rivalry.' Accordingly, in the 

 Protozoa, Echinoderms, Medusae, and Ctenophores, secondary sexual 

 characters are entirely absent, as pairing also is. 



In those worms that pair we first meet with secondary sexual 

 characters, and from this level upwards they are never quite absent 

 from any large group, and gradually play an increasingly important 

 rule. 



But the significance of sexual selection lies, as we have seen, not 

 only in the fact that one sex of a species, usually the male, is modified, 

 but in the possibility of the transference of this modification to the 

 females, and further, in the fact that the process of variation may 

 start afresh at any time, and thus one variation may be developed 

 upon or alongside of another. In this way we can explain certain 

 complex and often fantastic forms and colourings which we could not 

 otherwise understand; thus the extraordinary number of nearly related 

 species in some animal groups, such as butterflies and birds, in which 

 the differences mainly concern the colour-patterns. 



Darwin has shown convincingly that a surprising number of 

 characters in animals, from worms upwards, have their roots in sexual 

 selection, and has pointed out the probability that this process has 

 played an important part in the evolution of the human race also, 

 though, in this case, all is not yet so clearly and certainly known as 

 among animals. 



To conclude this section, I should like once again to call attention 

 to the deficiency which is necessarily involved in the assumption of 

 any selection, sexual selection included, namely, that the first 

 beginning of the character which has been intensified by selec- 

 tion remains obscure. Darwin attached importance to the occur- 

 rence of ordinary individual variation, but it is open to question 

 whether the insignificant variations thus produced could give an 

 adequate advantage in the competition for the possession of the 

 females; and, further, whether we have not grounds for the assumption 

 that larger variations also occur. This question may also be asked in 

 regard to ordinary natural selection, although in that case we can 

 imagine the beginnings to be smaller, since here the advantage of 

 a variation lies only in the fact that it is useful, not in its being 



