EVOLUTIONAL ZOOLOGY 217 



win and Spencer, as evidence of a similar transformation oc- 

 curring in nature. This argument was therefore of long stand- 

 ing when Darwin's work appeared, although he elaborated it 

 in far greater detail in his "Variation of Animals and Plants 

 under Domestication" than had any of his predecessors. 



4. The Succession of Organic Forms in Paleontology. — 

 The bearing of the paleontological facts upon the doctrine of 

 descent is too well known to need mention, and for many 

 years before the publication of the "Organic of Species" the 

 general parallelism between the appearance of organic forms 

 in the order of their complexity and the sequence of fossili- 

 ferous strata had been clearly recognized. The fact that the 

 regular succession of strata could be defined in terms of their 

 fossil content, and the observation that the fossils were more 

 divergent from modern forms the older the strata in which 

 they occurred had been thoroughly established by Werner and 

 William Smith, and on this paleontological evidence alone an 

 adequate basis for a doctrine of descent had been in existence 

 for over a quarter of a century before Darwin, Yet the geolo- 

 gists and paleontologists, although among the first of Darwin's 

 converts, had been unwilling to draw the obvious inference. 



5. The Recapitulation Theory. — The discovery of von 

 Baer, the founder of the science of embryology, that an indi- 

 vidual animal in its development as an embryo tends to pass 

 through a series of stages which roughly correspond with the 

 adult grade of organization of lower types, was interpreted 

 later in evolutionary terms, and expressed as the Law of Re- 

 capitulation — or the law that the successive stages in an indi- 

 vidual's development are a sort of resume of the stages through 

 which its ancestors have passed in their descent from less spe- 

 cialized forms. The significance of the facts upon which the 

 latter law was based was emphasized by Chambers with much 

 insistence, but his argument was regarded as groundless spec- 

 ulation and generally rejected as such, although a few years 



