288 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



form ever since the origin of the Amphibia l . Superfluous organs 

 become rudimentary very slowly, and enormous periods must elapse 

 before they completely disappear, while the older a character is, 

 the more firmly it becomes rooted in the organism. What I have 

 above called the physical constitution of a species is based upon 

 these facts, and upon them depend the tout ensemble of inherited 

 characters, which are adapted to one another and woven together 

 into a harmonious whole. It is this specific nature of an organism 

 which causes it to respond to external influences in a manner different 

 from that followed by any other organism, which prevents it from 

 changing in any way except along certain definite lines of varia- 

 tion, although these may be very numerous. Furthermore these 

 facts ensure that characters cannot be taken at random from the 

 constitution of a species and others substituted for them. Such a 

 variation as a mammal wanting the firm axis of the backbone is an 

 impossibility, not only because the backbone is necessary as a support 

 to the body, but chiefly because this structure has been inherited 

 from times immemorial, and has become so impressed upon the 

 mammalian organization that any variation so great as to threaten 

 its very existence cannot now take place. The view here set forth 

 of the origin of hereditary variability by amphigonic reproduction, 

 makes it clear that an organism is in a state of continual oscillation 

 only upon the surface, so to speak, while the fundamental parts of 

 its constitution, which have been inherited from extremely remote 

 periods, remain unaffected. 



Thus sexual reproduction itself did not cease after it had existed 

 in the form of conjugation through innumerable generations of the 

 vast numbers of species which have been included under the Protozoa; 

 it did not cease even when its original physiological significance 

 had lost its importance, either completely or in part. This process, 

 however, had come to possess a new significance which ensured its 

 continuance, in the enormous advantage conferred on a species by 

 the power of adapting itself to new conditions of life, a power which 

 could only be preserved by means of this method of reproduction. 

 The formation of new species which among the lower Protozoa 

 could be achieved without amphigony, could only be attained by 

 means of this process in the Metazoa and Metaphyta. It was only 



1 In frogs the sixth toe exists in the hind legs as a rudimentary prehallux. Com- 

 pare Born, Morpholog. Jahrbuch, Bd. I, 1876. 



