ADAPTATION 
27 
environment and the struggle for existence, and determine 
which are helpful, which hurtful, and to what extent. It is 
bound up with the study of geographical distribution, and 
intimately related to morphology on the one hand, physio- 
logy on the other 1 . 
We have now to return to morphology and consider its 
methods. The actual ancestral forms of any given plant 
being lost, other ways of tracing the phylogeny must be 
employed. 
Comparison with related forms is the method chiefly 
used in deducing phylogenies. If a given plant X be the 
immediate parent of several plants A, B, C, D , we know 
that these will resemble one another, as well as X , in detail, 
allowing for variation ; they are exactly equivalent to one 
another by descent. We now transfer this idea of equivalence 
to their organs, and say that those of A are equivalent by 
descent to the corresponding organs of B , C, D , or homo- 
logous with them, and we extend the idea to the ancestral 
forms, and say that they are also homologous with the 
corresponding organs of X , and with those of Y, a more 
remote ancestor, but the further back we go, the more change 
there may have been, and the less exact will the corre- 
spondence be. Even between A and B , the homology must 
not be pressed too closely into minute detail, e.g. while the 
primary roots may be exactly homologous, the hairs on the 
leaves will only be ‘generally ? homologous. 
If A, B, C, D are not the offspring of a single parent, 
but agree closely in all characters, we yet assume, on the 
general theory of descent, that they had a common ancestor, 
Y , not very far back. We therefore regard them as equivalent, 
and their corresponding organs as homologous. We further 
assume that the characters in which they agree exactly are 
characters derived from Y, and we can thus infer what Y 
was like. 
Now suppose that the lateral organs of the shoot in A 
are tendrils, in B thorns, in C and D leaves, while in all 
other characters the four agree. We may still say that the 
1 And see books mentioned under Variation ; Goebel, Organography 
of Plants ; Warming, Oekologische Pflanzengeographie ; Willis, Podoste- 
maceae , Ann. Perad. 1. 1903, pp. 417, 444, &c. 
