DEGENERATION AND LAWS OF VARIATION. 201 



the least developed eyes would save the most 

 energy, and so have an advantage over others in 

 the struggle for existence. This advantage, accord- 

 ing to that theory, would lead to the establishment 

 of a blind race, and the extinction of races with eyes. 

 When, however, we examine into the supposed 

 relation between the abundance of energy at the 

 disposal of an animal and the perfection of its 

 development, we find that the two conditions by 

 no means coincide. No animals have such an 

 abundance of energy at their disposal as those 

 parasitic animals which live in the bodies of their 

 hosts, continually bathed by nutritive juices, and 

 yet those animals have, above all others, degenerated 

 most in the growth and development of their parts. 

 They have abundance of energy at their command 

 for the performance of growth, development, and 

 all their functions, and yet from the time a race 

 begins a parasitic life, the evidences of its history 

 show that it constantly deteriorates. In some of 

 the crustaceans the organs of sense and locomotion 

 disappear, and the animal becomes a sac, incapable 

 of every function save nourishment and reproduc- 

 tion. Evidently, the abundance of readily converti- 

 ble energy is not the cause of a complete and perfect 

 development of an animal. And the fact that an 

 animal which is supplied only with an abundance 



