7O2 Studies in the Theory of Descent. 



brief to enable us to directly follow long series of 

 accumulations, whilst per saltum variation is ad- 

 mitted, although unsupported by a single obser- 

 vation ? As long as there remains any prospect 

 of tracing large deviations to the continually 

 observed phenomenon of small variations, I believe 

 we have no right to resort to the purely hypo- 

 thetical explanation afforded by per saltum vari- 

 ations. 



But the hypothesis of " heterogeneous genera- 

 tion " is not only without a basis of facts it can 

 also be directly shown to be untenable. Since 

 the operation of an internal power of transforma- 

 tion does not explain adaptation to the conditions 

 of life, the claims of natural selection to explain 

 these transformations must be admitted ; but the 

 co-operation of a phyletic vital force and natural 

 selection is inconceivable if we imagine the modi- 

 fications to occur per saltum. 



The supposed "heterogeneous generation" is 

 always illustrated by the example of alternation of 

 generation. The origination of a new animal 

 form is thus conceived to take place in the same 

 manner as we now see, in the cyclical reproduction 

 of the Medusae, free swimming, bell-shaped 

 Medusoids, produced from fixed polypites, or 

 Cercarics from Trematode worms by internal 

 budding ; in brief, it is imagined that one animal 

 form suddenly gives rise to another widely de- 

 viating form by purely internal causes, tyow on 



