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. 
Il THe Errects or 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 
