264 GENERAL BIOLOGY 



of interest here ; it begins with the maximum development 

 in size and complexity of leaves at sexual maturity, and, 

 passing through a diminishing series, ends with cessation 

 of leaf production when all the energies of the plant are 

 given over to the ripening of its seeds. 



Why evolutionary series? It has long been the custom of 

 naturalists to arrange organisms in series; such arrange- 

 ment facilitates dealing with large numbers. The compara- 

 tive anatomists of the first half of the iQth century, who did 

 so much to advance biological knowledge, believed in special 

 creation, and in the fixity of the species. They determined 

 homologies with great conscientiousness and arranged 

 organisms in natural groups; but for them, homology 

 meant likeness in structure merely, and not kinship, and 

 their groups were "natural" in the sense that like had been 

 associated with like in them. The organisms of a series 

 were no more related to each other than a series of one type 

 of vessels made by the same potter. Why then do we con- 

 sider that natural grouping signifies blood relationship? 

 Why are the series we arrange evolutionary series to us? 

 It is because evolution alone affords a consistent 

 and satisfactory explanation of the facts now known con- 

 cerning the structure, the development and the past history 

 of organisms. The student who has done the work hitherto 

 outlined will have felt this explanation. But perhaps it 

 may not be amiss to briefly indicate at this point a few classes 

 of facts that speak especially for evolution, and that seem to 

 stand in the way of any other explanation : 



1. The plasticity of species under domestication, and 



2. The intergradation of species in nature. 



Both these phenomena are well enough known to every 

 observing person and each shows that species are not fixed 

 and immutable. The individuals of a species may, there- 

 fore, be arranged in a series with its extremes having very 



