The Peinciples of Palaeontology. 153 



Advocates of the transf ormist doctrine had this hypothesis long 

 in mind, but only recently has it been definitely formulated, 

 and it is still at the present day difficult to give any very precise 

 proofs of it. We have thought it desirable to make particular 

 mention of it for the reason that it adapts itself in a remarkable 

 manner to palaeontologic data, and explains many difficulties. 



§ 2. Causes of Yaeiation.* 



Insufficiency of the theory of selection.^ Without recanting 

 any of the doctrines which he had so firmly established, Darwin, 

 toward the close of his life, became convinced that natural selec- 

 tion, all sufficient for the fixation of variations and the production 

 of divergencies, was not sufficient to explain the cause of these 

 variations, and was not the sole phenomenon which played a part 

 in the mechanism of evolution. More of a Darwinist than Dar- 

 win himself, Wallace has always attributed to natural selection 

 an exclusive influence; he admits that individual variations are 

 spontaneous, multifold and produced in every sense at hazard, and 

 that a very small proportion of them are transmitted by selec- 

 tion, and are of no utility in introducing any other force. 



One of the most interesting questions on the subject under 

 consideration is this, whether individual variations are really 

 spontaneous or whether they are due in some degree to the 

 direct action of the medium in which the organism exists. We 

 •know that Lamarck attributed a preponderating influence to 

 the conditions of the surrounding medium. He found the 

 explanation of the mechanisoi of variation in the development 

 of those organs which were frequently exercised, and the reduc- 

 tion of such as were not used. This is the phenomenon which 

 Ball calls, for the sake of brevity, the heredity of exercise. This 

 idea was, from the outset, ridiculed by prejudiced adversaries, 

 and judgment was passed on it, as is expressed by Isidore Geof- 

 froy Saint-Hilaire, " without any study having been made of the 

 sources themselves, and following unreliable accounts which are 

 to the views of Lamarck only what a caricature is to a portrait.'^ 



Darwin, and especially Wallace, at the outset rejected the 

 ideas of Lamarck without much investigation^of them, but they 

 have recently been revived with distinguished success by Herbert 



♦ Riley, On the Causes of Variation in Organio Forms. %,{Proc. Americ. Assoc. Adv. Sc. 1888.) 



20 



