438 The American Naturalist. [May, 
itary mechanism or merely upon the various stages of onto- 
geny. 
CONCLUSIONS. 
The general conclusion we reach from a survey of the whole 
field is, that for Buffon’s and Lamarck’s factors we have no 
theory of Heredity, while the original Darwin factor, or Neo- 
Darwinism, offers an inadequate explanation of Evolution. If 
acquired variations are transmitted, there must be, therefore, 
some unknown principle in Heredity ; if they are not trans- 
mitted, there must be some unknown factor in Evolution. 
As regards Selection, we find more than the theoretical ob- 
jections advanced by Spencer and others. Neo-Darwinism 
centers upon the principles of fortuitous variation, utility, and 
selection as universal. In complete fossil series it is demon- 
strated that these three principles, however important, are not 
universal. Certain new adaptive structures arise gradually, 
according to certain definite laws, and not by fortuity. 
Lamarck’s and Buffon’s factors afford at present only a par- 
tial explanation of these definite phylogenic variations, even 
if the transmission of acquired variations be granted. Niigeli’s 
factor of certain constitutional lines of variation finds consid- 
erable verification in fossil series as a principle of determinate 
variation, but not as a general internal perfecting tendency. 
St. Hilaire’s factor of occasional saltatory evolution by sudden 
modification of the hereditary mechanism is established, but 
not as yet understood, although we are perhaps approaching an 
explanation through experimental embryology. 
Our standpoint towards Variation in relation to all the Fac- 
tors requires thorough reconsideration. The Darwinian law 
of Fortuity and the Buffon law of the direct action of Environ- 
ment, have hitherto been inductions from variations which may 
be largely ontogenic and transitory. They both require con- 
firmation on data of phylogenic variation. As for Lamarck’s 
factor, the evidence seems to be conclusive that somatogenic 
variation is largely adaptive; but it remains to be proved that 
phylogenic variations as observed in human anatomy and in 
paleontology are invariably anticipated by corresponding 
