EXPLANATOKY HYPOTHESES 109 



evolution is definitely directed, proposed the hypo- 

 thesis of " Organic growth. " 26 He says that the con- 

 stitution of the organism determines the direction of 

 its development, and only a few directions are pos- 

 sible, so that indiscriminate modification is 

 prevented. However, through the agency of 

 external influences, the constitution of the organism 

 must be gradually changed. The outward as well 

 as the inward agencies do not act directly, but only 

 by being causes of growth ; and the external agencies 

 can only act through long persistence of the same 

 conditions. 



Mr. F. W. Hedley, in his "Problems of Evolu- 

 tion ' ' (1900) , advocates fission as the primary cause 

 of variation. It is inevitable, he says (p. 33), that 

 fission must at times be imperfect and that the two 

 cells formed by the division of the nucleus will 

 remain adhering to one another and so develop into 

 multicellular organisms. When a cell divides, the 

 two parts appear to be equal. But it is impossible 

 that they can always be exactly equal in size or 

 exactly alike in character. Hence fission must be 

 a source of variation. 



Criticisms and Suggestions. Such are the hypo- 

 theses. I have put them as clearly as I could, and 

 have tried to do full justice to each. I will now 

 examine them and see which offers the best explana- 

 tion of the phenomena. 



Lamarck made no attempt to explain heredity. 

 He took that for granted, and endeavoured to shew 



26 English translation in the "Religion of Science 

 Library," Chicago . 



