412 V. THE NATURE OF EVOLUTION 



readily. Thus a character acquired during one generation may disap- 

 pear rapidly on the removal of the causative effect. However, when 

 the effect continues to exist for generations to stabilize the acquired 

 character, the memory of the former structure will become faint, while 

 the new one being fixed. It may be supposed, therefore, that organs 

 like eyes having the tendency to degenerate rapidly are newly produced 

 organs retaining still the fresh memory of the original pattern. 



Attention has been paid for a long time to the fact that old organs 

 which were useful in the old environment but which have ceased to 

 have a function in the new, linger on in a reduced form with aston- 

 ishing persistence, the grip of ancestry still upon them. Various land 

 birds, particularly among those of oceanic island, have lost the power 

 of flight. Externally no trace of wing is visible, but close inspection 

 reveals a minute rudiment of wings still persisting under the long 

 hairlike plumages, although wholly without significance as an organ 

 of flight. Familiar examples of vestigial organs are presented by the 

 degenerative eyes of many cave animals. 



The writer believes that such vestigial structures were produced 

 as a result of a random change of the genes, or, more precisely, were 

 produced involuntarily being associated with the development of a 

 peculiar structure of a gene causing a certain useful character, and 

 when a certain structure thus produced involuntarily came to have the 

 value of use, the use would be commenced with the rapid development. 

 Therefore, the portion which is to be degenerated by the disuse must 

 be the structure that was developed rapidly by the use, vestigial 

 structure thus being left unchanged. 



Many useless structures are more fully developed in the embryo 

 than in the adult, thus presenting an instance of the recapitulation of 

 phylogeny by ontogeny. Some of the most striking cases are those of 

 functionless organs which develop in the embryo but which disappear 

 again before birth. For example, in whales both anterior and posterior 

 limbs are formed, though the latter subsequently atrophy and in some 

 species wholly disappear. Likewise the embryo is densely covered with 

 hair, although the adult whale is devoid of hair. Furthermore, it was 

 found in general that blind cave fishes develop what appear to be 

 normal eyes in their early life history, but later the eyes are lost again 

 through atrophy. 



If the individual develpoment is interrupted halfway, some char- 

 acter may remain in an embryonal state, a phenomenon called atavism. 

 The well known salamander, axolotl, shows atavism and remains for 

 life in an immature form, performing respiration by gills instead of 

 by lungs as in the larvae. This atavism is known as neoteny. Under 

 a peculiar condition associated with a food change or water deficiency. 



