Farmer.-— On the Structure of a Hybrid Fern. 541 
cell to one or other parent ; but the phenomena seem to be 
more readily susceptible of explanation if we assume that the 
hereditary substance acts as a whole , and by virtue of its own 
particular composition. The hereditary substance derived 
from two parents might equally well form a relatively stable 
or an unstable combination whether this depended on the 
nature of the molecular structure of the substance in question 
or on still more complex arrangement of molecular aggre- 
gates. In any case, operating, as it must, on and through the 
rest of the cell-contents, the results of its activity will be 
profoundly modified by the nature of the latter, and also, 
though less immediately perhaps, by such external agencies 
as in turn may affect them. And thus I think we may in 
a measure see how a plant can become materially altered 
in its form so as to suit different environments, without having 
to postulate the existence of a reserve store of emergency 
ids in order to explain the often extraordinary manifestation 
of adaptive variations which may arise within the limits of 
a single species or even of a single individual. In this con- 
nexion I might cite, for example, the variation of the leaf- 
form 1 met with in amphibious plants. We know of many 
cases in which a plant will develop in a particular way as 
the result of stimuli which we can hardly regard as other 
than physical or chemical, certainly not as the result of the 
direct action of hereditary ids. Thus pollination is a necessary 
antecedent to ovule-formation in many flowers. But if it be 
urged that the ids are there, and only need the stimulus to 
excite them to activity, this explanation can scarcely be 
seriously urged to meet the case of galls, each form of which 
is peculiar to and characteristic of a special parasite reacting 
on a special plant. Again, in the St. Valery apple, different 
forms of fruit are produced on the same tree as the result 
of pollination from different sources of apples. Here there 
can be no question as to the travelling out of the ids from the 
1 Weismann has discussed the question of polymorphism ( The Germ Plasm , 
Contemporary Science Series, 1893) with the result of adding considerably to the 
complexity of his theory. 
O O % 
