A Suggestion on JiJxtinctio7i. 477 



pare to a protozoan stage. The method of evolution along 

 different lines has been very aptly compared by Darwin to 

 the branches of a tree, and I agree with those who think there 

 is good evidence that, once a form takes to one branch, it 

 is limited in its path of evolution to that branch and its 

 smaller branches, and once it takes to one of the smaller 

 branches, it is limited to that branch and its buds to come. 

 There is apparently no anastomosing of the branches, and 

 no evolution from different stocks, converging to produce 

 the same result. 



To return to the one-celled primitive condition which lies 

 at the roots of the tree, we see that it must have had a 

 dormant power of variation, which, played on by natural 

 selection, has produced such different results as a plant, a 

 ccelenterate, an insect, a mollusc, a vertebrate, and so forth. 



On the other hand, an insect has no powers of evolving a 

 vertebrate as we understand them, nor a plant a mollusc. 

 They seem, therefore, in comparison with the primitive one- 

 celled condition, to have lost some of the possibilities of 

 variation. Let us say, therefore, that they have less 

 potential variation than the original one-celled condition. 

 There would then be a certain potential variation for every 

 group and every individual. 



This potential variation, also, would become more and 

 more limited within certain bounds at each stage in the 

 evolution of the different forms of life, from which there is 

 apparently only one method of return, and that with varying 

 possibilities. 



Even in the fauna, as it is at present constituted, there 

 appears to be greater possibilities of wide variation in 

 different directions in the lower forms of life than in the 

 higher, although the Protozoa even must have specialised to 

 a great extent since their original condition at the time they 

 were evolved. On the other hand, the actual variation 

 within the limits of possibility is apparently much more 

 rapid in the highly organised forms of life, and, indeed, the 

 more specialised they become, the more rapidly do they vary. 



The evidence of the higher groups of creatures always 

 having sprung from the more generalised forms in their 



