PEEFACE. 



It was Jaeger who once said — ^but I forget where — that 

 enough had been done in the way of philosophising by 

 Darwinists, and that the task that now lay before us was 

 to apply the test of exact investigation to the hypotheses 

 we had laid down. 



I have long been of the same opinion ; but it is in 

 fact a thing much easier said than done. It is infinitely 

 easy to form a fanciful idea as to how this or that fact 

 may be hypothetically explained, and very Uttle trouble 

 is needed to imagine some process by which hypothetical 

 fundamental causes — equally fanciful — may have led to 

 the result which has been actually observed. But when 

 we try to prove by experiment that this imaginary pro- 

 cess of development is indeed the true and inevitable one, 

 much time and laborious research are indispensable, or 

 we find ourselves wrecked on insurmountable difficulties. 



Nevertheless the step must be taken. The popular 

 cant about ' Biogenetic principles and the falsification of 

 Ontogenesis — the laws of inheriteince at corresponding 

 periods of life, or the correlation of organs — Ontogeny 



