6 HEREDITY, AND THE ORIGIN OF SPECIES. 
ity, or to environment. These rudimentary organs may, 
in the course of generations wax in size, undergo increas- 
ing differentiation of structure, perhaps finally becoming 
sub-functional or even fully functional, although nascent 
organs are not always supposed to attain this useful end. 
In support of this theory it has been pointed out that plants 
and animals show many structures, which so far as our 
understanding of them goes, are wholly without part in 
the life of the organism. The present development of plant 
morphology, however, is one which is carrying us farther 
and farther away from the conception of such pre-func- 
tional formation of organs, as the whole tendency of mod- 
ern investigation is to place the morphogenic processes 
upon a physiological basis. On the other hand the argu- 
ment that the variations of the organisms are “determi- 
nate,” and are governed by the morphological possibilities 
is one which surely holds for any method of phylogenetic 
procedure. Thus it needs but the briefest common-sense 
consideration to show that any given type of leaf or flower, 
could not possibly vary toward all other types, but only in 
the direction of certain forms not too widely different. 
A more explicit statement may prevent a misconcep- 
tion. It has been held by some writers that variations and 
mutations, theoretically follow those already made, as 4 
projection of them, or a continuation, which carries the 
organs concerned, successively and ever nearer some ideal 
form or type. This is determinate variation in its strictest 
sense. On the other hand, it is argued that from any given 
stage in its development an organ may vary or mutate in 
any direction limited of course by its morphological possi- 
bilities, and such alteration may lead the structures con- 
cerned in any given course from that previously pursued. 
It need only be said that to the experimenter the latter 
view seems to be the more fully justifiable by the facts ob- 
served in mutations. 
