230 THE HISTORY OF CREATION. 
parental organism, the egg of the mother or the sperm of 
the father. 5 
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 effect 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 greatest inportance. 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 
