230 THE HISTOEY OF CREATION. 



parental organism, the egg of the mother or the sperm of 

 the father. 



A third curious manifestation of indirect adaptation may- 

 be termed the law of sexual adaptation. Under this name 

 we indicate the remarkable fact that certain influences, 

 which act upon the male organs of propagation only, affect 

 the structure of the male descendants, and in like manner 

 other influences, which act upon the female organs of propa- 

 gation only, manifest their efiect only in the change of struc- 

 ture of the female descendants. This remarkable pheno- 

 menon is still very obscure, ^and has not as yet been 

 investigated, but is probably of great importance in regard 

 to the origin of " secondary sexual characteristics," to which 

 we have already made allusion. 



All the phenomena of sexual, monstrous, and individual 

 adaptation, which we may comprise under the name of the 

 laws of indirect or ^potential adaptation, are as yet very 

 little known to us in their real nature and in their deeper 

 causal connection. Only this much we can at present main- 

 tain with certainty, that numerous and important trans- 

 formations in organic forms owe their existence to this 

 process. Many and striking variations of form solely de- 

 pend on causes which at first only affect the nutrition of the 

 parental organism, and specially its organs of propagation. 

 Evidently the relations in which the sexual organs stand to 

 other parts of the body are of the gTeatest importance. We 

 shall have more to say of these presently, when we speak of 

 the law of correlative adaptation. How powerfully the 

 variations in the conditions of life and nutrition affect the 

 propagation of organisms is rendered obvious by the re- 

 markable fact that numerous wild animals which we keep 



