248 Influence of environment on animals 



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

 chemical agencies, temperature, light, and gravitation. We shall also 

 treat separately the effect of these forces upon form and instinctive 

 reactions. 



II. The Effects of Chemical Agencies. 

 (a) Heterogeneous hybridisation. 



It was held until recently that hybridisation is not possible except 

 between closely related species and that even among these a successful 

 hybridisation 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 unfertilised 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 guarantee that the eggs are fertilised, for the sper- 

 matozoa are carried to the eggs by chance and are not attracted 

 by the latter. This statement is the result of numerous experi- 

 ments 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 been shown by 

 experiment that in fresh sea- water the sperm may live and retain its 

 fertilising power for several days. It is thus unavoidable that at 

 certain periods more than one kind of spermatozoon is suspended in 

 the sea- water and it is a matter of surprise that the most heterogeneous 

 hybridisations do not constantly occur. The reason of this becomes 

 obvious if we bring together mature eggs and equally mature and 

 active sperm of a different family. When this is done no egg is, as 

 a rule, fertilised. The eggs of a sea-urchin can be fertilised by sperm 

 of their own species, or, though in smaller numbers, by the sperm of 

 other species of sea-urchins, but not by the sperm of other groups of 

 echinoderms, e.g. starfish, brittle-stars, holothurians or crinoids, and 

 still less by the sperm of more distant groups of animals. The 

 consensus of opinion seemed to be that the spermatozoon must enter 

 the egg through a narrow opening or canal, the so-called micropyle, 

 and that the micropyle allowed only the spermatozoa of the same or 

 of a closely related species to enter the egg. 



It seemed to the writer that the cause of this limitation of 

 hybridisation might be of another kind and that by a change in the 

 constitution of the sea-water it might be possible to bring about 

 heterogeneous hybridisations, which in normal sea-water are im- 

 possible. This assumption proved correct. Sea-water has a faintly 

 alkaline reaction (in terms of the physical chemist its concentration 



