200 DR W. B. DRUMMOND ON 
gradual improvement of structure, and for the evolution of 
higher and higher types. As the struggle for existence 
became more keen, some forms sought salvation by a return 
to a free-swimming life; and so the bottom, and the shore, 
and the land have all at various times given contributions 
to the pelagic fauna. The types first to return to a surface 
life must have been such as were able to subsist on the 
minute plants and animals of the surface, but they in turn 
were fated to serve as food for yet higher forms that sought 
safety in the surface waters. The evolution of these shore 
and bottom forms that thus by degrees contributed to the 
pelagic fauna must, as we have seen, have taken place in 
pre-Cambrian times. Mr W. K. Brooks (to whose writings 
I am indebted for many hints on this subject) believes that 
when colonisation began on the bottom and the shore, the 
evolution of metazoan types may have proceeded for a time 
with comparatively great rapidity, on account of the diver- 
sity of the conditions of existence, and the greater “ hardness ” 
of these conditions as compared with the surface, and that 
in this way we may be helped to understand the very early 
appearance of the principal invertebrate types. 
Thus, then, we see that all classes of the animal kingdom 
have contributed their quota to the fauna of the surface. 
These as they increased in numbers gradually effected an 
important alteration in the conditions of existence at the 
surface. The inorganic conditions are, as we have seen, 
of remarkable uniformity, but the appearance of numerous 
competing types of organisms introduced a new element, and 
served to bring about further evolution and specialisation. 
Of the many larval forms which abound in the surface 
waters, a few may be regarded as secondary in character. 
That is to say, they are the modified young of more or less 
sedentary bottom-loving forms which have acquired motile 
powers for the purpose (to make use of a teleological 
expression) of diffusing the species. Most of the free- 
swimming larve may rather be regarded as representing 
primitive types, which have been retained on account of the 
benefit to the species of the wider diffusion thus brought 
about. Even so, most of them have undergone secondary 
adaptive changes. 
