70 THE TRANSFORMATIONS OF THE ANIMAL WORLD 



one of the characteristics of this type in the series 

 called regressive, the new types are always distin- 

 guished from the primitive types by special features 

 easy to recognize. 



The extreme rarity of really continuous series 

 has led some naturalists to suppose that, in many 

 cases at least, the transition from one form to another 

 or from one series of mutations to another series 

 must have occurred by sudden leaps. Perhaps also 

 we ought to think that the development of the 

 series has been characterized by short periods of 

 rapid changes separated by longer periods of relative 

 steadiness. But the slow and gradual variation 

 of species remains none the less established by a 

 complete series of certain proofs. 



If, now, we desire to carry further the problem 

 of the great transformations which took place in 

 geological times, we shall no longer find proofs as 

 immediate as those we have pointed out in the 

 narrow field of the formation of species, thanks to 

 the series of natural forms and to the experiments 

 on domestic races. Palaeontology nowhere shows us 

 a series of transitional types between distant groups, 

 as would be, for instance, a series going from the 

 Protista to the Mammals ; we shall here have to 

 content ourselves with conclusions from analogies 

 and with proofs by probabilities, which otherwise 

 will not be wanting in the very different groups of 

 fossil animals. 



A first comprehensive glance at the succession 

 of fossil faunas shows a continuity very favourable 

 to the descent hypothesis. If, leaving the present 

 world, we plunge into more and more ancient strata, 



