PALEONTOLOGY 93 



period lo period or from stratum to stratum, species 

 suddenly appear and disappear, apparently without 

 ancestors and without descendants. 



According to the theory of evolution, and espec- 

 ially of natural selection, if we start with any organ- 

 ism and trace its history backward, we would find 

 that through an endless number of generations it had 

 been very slightly changing, so that any individual is 

 always a transitional form between its immediate 

 ancestors and its own offspring. 



If, according to this, we had all of man's ancestors 

 in line, they would extend by almost imperceptible 

 differences of form from Protozoan to Man. About 

 5,500 species of mollusks have been found in the 

 Silurian age, in which all the classes of mollusks are 

 largely represented. The shells of mollusks being 

 composed of carbonate of lime are extremely durable, 

 and, as fossils, they compose a large part of most 

 limestones. This being true, one would expect, if the 

 theory of evolution is true, to find vast numbers of 

 transitional forms connecting earlier and later species 

 in the various periods where fossils are well pre- 

 served. This, however, is not true. Species, when 

 they first appear, stand sharply defined. 



Darwin expresses his disappointment at the absence 

 of transitional forms as follows: " But I do not pre- 

 tend that I should ever have suspected how poor was 

 the record in the best preserved geological sections, 

 had not the absence of innumerable transitional links 

 between the species which lived at the commencement 

 and close of each formation pressed so hardly on my 

 theory." * 



Taking the mollusks of the Silurian, it is evident 

 that the transitional forms representing slight succes- 

 sive changes must have almost infinitely outnumbered 



* Origin of Species, p. 286. 



