MEMO a oin a aia 
MELANTHACEÆ FROM THE GENETIC STANDPOINT. 133 
in structural characteristies often co-exist side by side * indicates that these 
differences were probably not of selective value, but they arose through what 
Darwin called “spontaneous variation,’ and have been perpetuated by 
heredity. It is thus believed that, while natural selection accounts for the 
gradual development of adaptations through the accumulation of larger or 
smaller variations, the mutation theory accounts for the origin of innumerable 
generic and specific differences which were never of selective value to their 
possessor. In the years when natural selection was the only accepted factor 
of evolution, it always requiredalarge measure of credulity to believe that every 
character of every organism had been at some time of particular value to its 
possessor. The mutation theory renders unnecessary such an assumption, 
and thus relieves natural selection of the necessity of explaining too much. 
With these and similar conceptions in mind, the North American Melan- 
thaceze have been studied systematically. The result is incomplete, because 
some of these genera are still insufficiently known, and I have not had an 
unlimited time to devote to the subject. It is hoped that the work will be 
most useful for its suggestiveness. We know that many variations are 
marked steps, and that such variations are inherited, while we as yet know 
very little of the inheritance of continuous variations, and what is known is 
largely negative in character. If systematists are to reflect the knowledge 
of their time in the construction of their species, then obviously they need 
to pay at least as great attention to discontinuous as to continuous variability, 
our knowledge of the former being firmly based on experiment. 
The analyses of specific and generic differences attempted in this paper 
may appear speculative, but at least they are no more so than they would 
be if they assumed variation to be continuous. They, moreover, have the 
definite advantage of being based on analogy—sometimes direct analogy— 
with known experimental results. It is hoped that the explanations here 
suggested of various specific and generic relationships will lead to the further 
study of many of these species both eytologically and experimentally, for this 
is the only way in which the questions here propounded can be definitely 
solved. It would be an attractive problem to make a comparative study of the 
chromosomes throughout the group, and it is hoped that botanists will avail 
themselves of this opportunity whenever they can obtain the necessary material. 
In the systematic treatment of these groups it has been necessary to 
describe several new species and varieties and to make several new com- 
binations, though an effort has been made to circumscribe their number. 
It is perhaps doubtful whether the segregation from Zigadenus of the genera 
T'oxicoscordion, Anticlea, and particularly Oceanorus should be recognized, 
though I have here given them the benefit of the doubt. Tt is easily 
* Cf. Gates, R. R., “The Mutation Theory and the species-concept," in Amer. Nat., 
Oct. 1917. 
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