LECTURE XXXIII 



ORIGIN OF THE SPECIFIC TYPE 



Transition species of Celebes snails, according to Sarasin — Possible variations in 

 the shell due to nutrition— Natural selection plays a part— Germinal selection- 

 Temporary transitions between species— The fresh-water snails of Steinheim— How do 

 sharply-defined species arise?— Nageli's Developmental Force— The species a complex 

 of adaptations— Adaptive differences between species— Adaptive nature of specific 

 characters— The case of Cetaceans— Of birds— Additional note : the observations and 

 theories of De Vries. 



OuE study of the influence which geographical isolation may 

 have in transforming old and giving rise to new forms of life has 

 led us naturally to a much more important problem, that of the 

 origin of species as more or less sharply defined groups of forms, 

 and I wish to make the transition to this problem by discussing 

 another case of species-sp)litting effected in association with, or, as 

 is usually said, through isolation. The naturalists Paul and Fritz 

 Sarasin, well known through their excellent studies on many com- 

 ponents of the tropical fauna, have published in their latest work 

 interesting discoveries in regard to the terrestrial snails of Celebes. 

 These observations show that on this island a great transformation 

 of snails has taken place, even since the later Tertiary period. A large 

 number of new species of snail have arisen on this island since that 

 time, and this, as the authors show to be probable, in association with 

 the receding of the sea, that is, with the elevation of the island further 

 out of the water, and thus with the increase of its surface. The 

 modern terrestrial snails show chains of forms connected in many 

 ways so that a series of species is connected by transition forms, and 

 therefore does not really consist of separate species at all, although 

 the extremes would seem to be separate species if they were 

 studied by themselves without taking the transition forms into 

 account. The state of things is exactly as if a Tertiary snail had 

 spread from any small area over the whole island, and had been 

 transformed slowly and in a definite direction in accordance with its 

 distance from its starting-point. It is thus that we must interpret 

 this discovery ; we have here, beside each other in space, and indeed 

 often disposed along geographical lines, the individual stages of a 

 phyletic process of transformation, which has reached different levels 



