APPLICATIONS It 
ecology” and in its scientific aspects, which are its foundation, it is precisely 
the ecology of woody plants, and of the vegetation which they constitute. 
Apart from botany, the physical side of ecology is largely a question of soil 
physics, and of physiography. On the other hand, vegetation is coming 
more and more to be regarded as a fundamental factor in soogeography 
and in sociology. Furthermore, with respect to the latter, it will be pointed 
out below that the principles of association which have been determined for 
plants, viz., invasion, succession, zonation, and alternation, apply with almost 
equal force to man. 
a 
17, Physiology and pathology. The effect of ecology in emphasizing the 
intrinsically close connection between physiology and morphology has al- 
ready been mentioned. Its influence in normalizing the former by forcing 
it into the field as the place for experiment, and by directing the chief at- 
tention to the plant as an organism rather than a complex of organs, is also 
rapidly coming to be felt. Ecology will doubtless exert a corrective in- 
fluence upon pathology in the near future. This is inevitable as the latter 
ceases to be the merely formal study of specific pathogenic organisms, and 
turns its attention to the cause of all abnormality, which is to be found in 
the habitat, whether this be physical, as when the water-content is low, or 
biotic, when a parasitic fungus is present. The relative ease with which 
specific diseases can be studied has helped to obscure the essential fact that 
the approach to pathology must be through physiology. Much indeed of 
the observational physiology of the laboratories has been pathology, and it 
will be impossible to draw a clear line between them until precise experi- 
ment in the habitat has come into vogue. 
18. Experimental evolution. As a result of the extremely fragmentary 
character of the geological record, nothing is more absolute than that there 
can be no positive knowledge of the exact origin of a form or species, ex- 
cept in those rare cases of the present day, where the whole process has 
taken place under the eye of a trained observer. The origin of the plant 
forms known at present must forever lie without the domain of direct knowl- 
edge. If it were possible by a marvel of ingenuity and patience to develop 
experimentally Myosurus from Selaginella, this would not be absolutely 
conclusive proof that Myosurus was first derived in this way. When all 
is said, however, this would be the very best of presumptive evidence. It 
must also be recognized that this is the nearest to complete proof that we 
shall ever attain, and with this in mind it becomes apparent at once that 
evidence from experiment is of paramount importance in the study of evo- 
lution (the origin of species). 
