ORGANIC EVOLUTION — THE FACTORS 55 



has been partly or mainly brought about by the circum- 

 stance that the animals have lived under such conditions 

 as constantly caused them to ruri as swiftly as possible ; 

 which endeavours at swift running caused in the indi- 

 vidual hares a development, as in athletes, of the 

 structures that subserve swift running, and that this 

 acquired development, transmitted to the offspring, and 

 accumulated in the course of generations, has been a 

 part or a main cause of the evolution of speed in hares. 

 To take another example ; the muscles of a blacksmith's 

 arm are enlarged by exercise. It is contended by the one 

 school that this enlargement, being an actpiired not an 

 inborn variation, is not at all transmitted, bvthe other that 

 it is in part transmitted. To take a third example; suppose 

 a man to be enfeebled by disease. It is contended by 

 the one school that this acquired enfeeblement cannot be 

 transmitted ; by the other that it can and is transmitted. 



The theory that evolution has resulted from the accu- 

 mulation of inborn variations alone is usually, but incor- 

 rectly, called the theory of evolution by Natural Selection. 

 I say incorrectly, for if acquired variations are trans- 

 missible, they must equally wath inborn variations be 

 seized upon and accumulated by Natural Selection ; 

 and therefore a theory that supposes that the organic 

 world has arisen by the accumulation of acquired varia- 

 tions is as much a theory of evolution by Natural 

 Selection, as a theory that supposes that it has arisen 

 by the accumulation of inborn variations alone ; par- 

 ticularly since variations acquired in one generation 

 become, if transmitted, inborn in the next generation. 



The theory that acquired traits are transmissible is 

 older than the theory which denies their transmissi- 

 bility. Lamark, finding evidence of evolution in the 

 organic world, published in the beginning of this cen- 

 tury a work in which he attributed all evolution to the 

 accumulation of acquired traits. In 1858 Darwin and 



