286 EVOLUTIONARY BEARINGS OF THE RESULTS [ch. 



three of these is proved by the facts of Palaeontology. There is reason to 

 believe that those main phyla, already characterised as they were in Palaeo- 

 zoic or in early Mesozoic time, have maintained their separate identity to 

 the present day, through persistent genera or their direct derivatives. Wide 

 homoplastic advance has affected these several phyla independently of one 

 another. Its results have been found to constitute a great Class-Progression 

 or Phyletic Drift, expressed in many apparently independent structural 

 changes. It leads from the Eusporangiate state characteristic of Palaeozoic 

 time, by a plurality of distinct lines of detailed modification, to the advanced 

 Leptosporangiate state characteristic of Present-Day Ferns. 



How from the evolutionary point of view are we to regard such a Class- 

 Progression, carried out in a plurality of distinct lines, and involving mafiy 

 heritable changes which can only have been achieved independently of one 

 another in the several phyla which show them? We may take refuge in the 

 beneficent word "adaptation," and may exercise ingenuity in accounting for 

 what we see by fitting effect with supposed cause. But it would be better 

 to attempt some more searching analysis of the influences effective in pro- 

 ducing the evolutionary results which we see. The most important of these 

 operative in the evolution of living things may be grouped under four 

 heads: 



(i) A general initiative present in all organic life to develop. 



(ii) Stimuli and limiting factors that shape and control the results. 



(iii) Syngamy and Mendelian Segregation that distribute those 

 results. 



(iv) Natural Selection which determines survival or failure. 



Of these (iii) and (iv) need not be discussed here, for they do not produce 

 new features: they only distribute, select, or annihilate them. The immediate 

 interest will lie in the origin and modification, rather than in the manipula- 

 tion or destination of characters. Here, therefore, we need only consider 

 (i) and (ii). 



The consequences of the all-pervading initiative of living things to develop 

 are seen primarily in an increase of size of the individual: secondly, in an 

 increasing complexity of its form and structure: thirdly, in its variation of 

 detail as shown by comparison of individuals and races. It is upon these 

 variations that the theory of evolution is based, and the question at once 

 arises of their origin and transmission. On the latter point variations have 

 been ranged in two categories: {a) fluctuating variations which appear to 

 leave no permanent impress so as to affect the reproductive cells; conse- 

 quently they are held as not being hereditary: {b) mutations which are 



