APPLICATIONS 1 1 



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 zoogeography 

 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. 



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). 



