xxii TRANSLATOR'S PREFACE 



Nature has embraced the principles of Weismaiin's Neo- 

 Darwinism, and while willing to devote plenty of space to 

 favourable reviews of Weismann's essays written by under- 

 graduates, suppresses without a word of explanation or apology 

 contributions which argue against the fashionable creed. The 

 argument rejected is as follows : According to the followers 

 of Weismann, evolution is due to the selection and " sexual 

 mixture " of congenital variations, that is to say, of variations 

 themselves due to sexual mixture, and having nothing to do 

 with the physiological effects of conditions or stimuli or 

 functional activity. These congenital variations, in animals 

 which are hatched or born, must either be present at 

 hatching or birth, or must appear at a later period of life. 

 If they are supposed to be present at hatching or birth, 

 then they can have nothing to do with any of the most 

 important and most interesting modifications which w^e know 

 to have taken ]3lace in the evolution of animals. For such 

 modifications do not begin in individual development until 

 long after birth or hatching. For instance, flat-fishes when 

 first hatched are perfectly symmetrical, and their metamor- 

 phosis does not begin till they are some weeks old. On 

 the other hand, if the disciples of Weismann say that 

 congenital variations appear at later periods of life after 

 birth or hatching, how are such variations to be distinguished 

 from acquired characters due to stimulus and functional 

 activity ? Can it be proved, for example, that the first 

 slight asymmetry of the eyes of the ancestral flat-fish which 

 appeared long after hatching, in a fish which crouched on 

 the ground on one side, was not due to the use that 

 particular fish made of its eyes ? Is there any criterion 

 by which we can distinguish among variations appearing 



