EVIDENCE FROM EXPERIMENT 151 



turned to their original type. The same species of 

 Ranunculus may be grown on land or in the water, 

 with totally different forms of leaf in the two media. 

 The land individuals have broad, tripartite leaves, 

 with serrate edges, while in the water plants the 

 leaves are subdivided into long, thread-like fila- 

 ments and the whole appearance of the plant is 

 that of a different species. "In all these cases, the 

 plant has not inherited a definite form from its 

 parents, but, within the limits of the hereditary 

 fundamental plan of structure, it has received cer- 

 tain possibilities of development, which did not 

 appear in the parents. Which of these possibilities 

 shall be realized in the development of the indi- 

 vidual, is determined by external conditions." 1 



This must not to be taken to mean that it is not 

 possible to produce hereditable modifications in 

 plants, the contrary is true, but merely that very 

 striking changes may be of no lasting importance 

 because they are not transmissible. 



Of late years, a host of experiments have been 

 performed upon animals, the larger number of them 

 with the object of determining whether new char- 

 acters, acquired during the lifetime of the parents, 

 can, under any circumstances, be transmitted to the 

 offspring. This is one of the most hotly disputed 

 questions of modern biology and our whole concep- 

 tion of the efficient factors which have brought about 

 evolution hinges upon the answer to this question. 



1 Karl Giesenhagen, in Die Abstammungslehre, pp. 307-8. 



