236 THE PLAY OF ANIMALS. 



the fifth cranial nerve. Supposing this to be a fact, 

 still nothing has been said that is prejudicial to the 

 theory of sexual selection — it must necessarily have 

 some sort of physiological basis. However, I for one 

 can not quite conceive how such developments as, for 

 instance, a peacock's tail, can be derived from begin- 

 nings so insignificant, simply by a superabundance of 

 energy. This is very delicate ground, for the hypotlie- 

 sis of surplus energy continuing through thousands of 

 generations seems to me to accord little with the laws of 

 natural selection, which are like the old laAvs of reward: 

 they give with niggardly hand what is essential for the 

 preservation of the species and no more.* 



However, Wallace thinks that such extraordinary 

 developments occur only when the species has acquired 

 an assured position in life — in fact, " perfect success in 

 the struggle for existence. . . . The enormously length- 

 ened plumes of the bird of paradise and of the peacock 

 are rather injurious than beneficial in the bird's ordi- 

 nary life. The fact that they have been developed 

 to so great an extent in a few species is an indication 

 of such perfect adaptation to the conditions of exist- 

 ence, that there is in the adult male at all events a 

 surplus of strength, vitality, and growth power, wdiich 

 is able to expend itself in this way without injury.^^ f 



But it is a well-known fact and a legitimate deduc- 

 tion from the principle of selection that such perfect 

 adaptation to surrounding conditions produces a fixed 

 type and precludes further development, just in propor- 

 tion to its perfection. Thus, even if we suppose that 



* The occasional surplus of energy arising from alternate waste 

 and reintegration is. of course, quite another thing, 

 f Ibid., pp. 292, 293. 



