xv i Preface to the English Edition. 



cally justifiable hypothesis of the origin of the 

 organic world. 



The materials accumulated in all the provinces 

 of biology now for the first time acquired a deeper 

 meaning and significance ; unexpected inter-rela- 

 tions revealed themselves as though spontaneously, 

 and what formerly appeared as unanswerable 

 enigmas now became clear and comprehensible. 

 Since that time what a vast modification has the 

 subject of animal embryology undergone ; how 

 full of meaning appear the youngest develop- 

 mental stages, how important the larvae ; how 

 significant are rudimentary organs ; what depart- 

 ment of biology has not in some measure become 

 affected by the modifying influence of the new 

 ideas ! 



But the doctrine of development not only 

 enabled us to understand the facts already 

 existing ; it gave at the same time an impetus to 

 the acquisition of unforeseen new ones. If at the 

 present day we glance back at the development 

 of the biological sciences within the last twenty 

 years, we must be astonished both at the enormous 

 array of new facts which have been evoked by the 

 theory of development, and by the immense series 

 of special investigations which have been called 

 forth by this doctrine. 



But while the development theory for by far the 

 greater majority of these investigations served as 

 a light which more and more illuminated the 



