I] HOMOLOGY 13 



as to produce the marvellous variety of species inhabiting the globe, 

 the only difference between human and animal colonies being that, 

 in the latter case, the divergence has become so great that animal 

 colonists will no longer breed with the original race. Thus, accepting 

 Darwin's theory, we find it possible to give a rational explanation of 

 those resemblances between animals which are expressed in a system 

 of classification 1 . If the theory be rejected these resemblances are 

 pure figments of the human mind, and the species must be regarded 

 as just as independent of one another as are the chemical atoms. 

 Hence since it is a choice between this explanation or none, the 

 Darwinian theory of gradual evolution is accepted by the over- 

 whelming majority of naturalists. Differences however exist as to the 

 nature and origin of the variations out of which evolutionary change 

 is built up. It has been shown that the minute differences between 

 brothers and sisters on which Darwin relied are usually non-inheritable. 

 Larger variations occurring at rarer intervals are strongly inherited 

 but as already mentioned these are of the nature of pathological 

 defects and are utterly unlike the marks which divide natural species. 

 Of quite recent years some evidence has been brought forward to show 

 that increased use which leads to increased size produces inheritable 

 effects. The selection would then operate in causing the survival 

 of those that responded most actively to the needs imposed by the 

 environment. 



One or two interesting consequences follow from the acceptance 

 of this theory. The structural features of animals are to be regarded 

 as adaptations to their surroundings, since they have been built up 

 by natural selection. Hence an isolated resemblance in a particular 

 feature between two species need not necessarily indicate that this 

 feature was present in the common ancestral species, for similar 

 surroundings may have evolved a similar modification in two 

 animals only remotely related. Such similarities are called homo- 



1 Most of the names employed in classification were in use before Darwin's 

 views were accepted. The word phylum (Gr. QvKov, tribe or stock) is however 

 an exception. This term expresses the central idea of the evolution theory, 

 and its proper use is to denote the whole of a group of animals characterised 

 by having the same ground-plan of structure and believed to be the descendants 

 of a common ancestor, from whom no other living animals are descended. 

 The essential feature about a phylum is its isolation, in the present state 

 of our knowledge, from other phyla. Of course it is believed that at bottom 

 all living beings constituted one phylum, but there are enormous differences 

 in structure which can only be bridged by imaginative hypotheses. 



