114 
DARWINISM 
CHAP. 
amounts, there seems no reason to believe that the necessity 
for two or more coincident variations would prevent the 
required change from taking place. 
The Continued Existence of Low Forms of Life. 
Since species are continually undergoing modifications 
giving them some superiority over other species or enabling 
them to occupy fresh places in nature, it may be asked—Why 
do any low forms continue to exist ? Why have they not long 
since been improved and developed into higher forms ? The 
answer, probably, is, that these low forms occupy places in 
nature which cannot be filled by higher forms, and that they 
have few or no competitors; they therefore continue to 
exist. Thus, earthworms are adapted to their mode of life 
better than they would be if more highly organised. So, in 
the ocean, the minute foraminifera and infusoria, and the 
larger sponges and corals, occupy places which more highly 
developed creatures could not fill. They form, as it were, the 
base of the great structure of animal life, on which the next 
higher forms rest; and though in the course of ages they 
may undergo some changes, and diversification of form 
and structure, in accordance with changed conditions, their 
essential nature has probably remained the same from the 
very dawn of life on the earth. The low aquatic diatomacege 
and conferva?, together with the lowest fungi and lichens, 
occupy a similar position in the vegetable kingdom, filling 
places in nature which would be left vacant if only highly 
organised plants existed. There is, therefore, no motive 
power to destroy or seriously to modify them; and they have 
thus probably persisted, under slightly varying forms, through 
all geological time. 
Extinction of Lower Types among the Higher Animals. 
So soon, however, as we approach the higher and more 
fully developed groups, we see indications of the often re¬ 
peated extinction of lower by higher forms. This is shown 
by the great gaps that separate the mammalia, birds, reptiles, 
and fishes from each other ; while the lowest forms of each are 
always few in number anil confined to limited areas. Such 
