EVOLUTIONARY THEORIES SINCE DARWIN 235 : 
duce their own fit kind. Many biologists since his 
time have doubted whether unaided Natural Selec- 
tion will account for the constant advance in organ- 
isms. This is the part of the work which is often 
seriously questioned. 
Weissman and his co-workers have contended that 
this unaided principle will serve. Most biologists have 
asked for some more efficient cause, and assert that se- 
lection does not account for the appearance of varia- 
tions, but only for their preservation, and that any 
valid theory of evolution must show how variations 
originate. It is chiefly in this respect that Darwin’s 
work has failed to satisfy many later biologists. 
When we hear a scientist speak of Darwinism as be- 
ing dead, this is what he means. He does not think 
evolution false, but believes that Natural Selection is 
not sufficient to account for evolution. There are 
three main difficulties involved in Darwin’s theory. 
The chief defect lies in the fact that selection cannot 
originate varieties. In all his earlier works Darwin 
simply accepted variations as he found them. He was 
content to say that all species varied constantly, and 
in every direction. He gave no theory to account for 
variation. Whenever he took measurements of the 
dimensions of any large series of objects of the same 
kind he found these measurements to vary, apparently, 
in all directions. Upon the facts of these variations, 
