196 The Mechanistic Conception of Life 



not mentioned here were considered to be of minor importance. 

 All the writer could hope to do was to bring together a few 

 instances of the experimental analysis of the effect of environ- 

 ment, which indicate the nature and extent of our control 

 over life phenomena and which also have some relation to the 

 work of Darwin. In the selection of these instances preference 

 is given to those problems which are not too technical for the 

 general reader. 



The forces, the influence of which we shall discuss, are in 

 succession chemical agencies, temperature, light, and gravita- 

 tion. We shall also treat separately the effect of these forces 

 upon form and instinctive reactions. 



II. THE effects of CHEMICAL AGENCIES 



a) Heterogeneous hybridization. — It was held imtil recently 

 that hybridization is not possible except between closely related 

 species and that even among these a successful hybridization 

 cannot always be counted upon. This view was well supported 

 by experience. It is, for instance, well known that the majority 

 of marine animals lay their uofertilized eggs in the ocean and 

 that the males shed their sperm also into the sea-water. The 

 numerical excess of the spermatozoa over the ova in the sea- 

 water is the only guaranty that the eggs are fertilized, for the 

 spermatozoa are carried to the eggs by chance and are not 

 attracted by the latter. This statement is the result of numer- 

 ous experiments by various authors, and is contrary to common 

 belief. As a rule all or the majority of individuals of a species in 

 a given region spawn on the same day, and when this occurs the 

 sea-water constitutes a veritable suspension of sperm. It has 

 recently been shown by experiment that in fresh sea-water the 

 sperm may live and retain its fertilizing power for several days. 

 It is thus unavoidable that at certain periods more than one kind 

 of spermatozoa is suspended in the sea-water and it is a matter 

 of surprise that the most heterogeneous hybridizations do not 



