EVOLUTION, VARIATION AND HEREDITY 465 



and trustee for the germ cells, and this idea may be expressed roughly 

 by a diagram. Weissmann points out that no satisfactory way has 

 been discovered whereby a subsequent alteration in such organs 

 could conceivably influence the germ cells from which they have 

 long been dissociated. Many modern Biologists are in general 

 agreement with this view, which they claim is supported by the 

 evidence of recent cytological and embryological investigations, but 

 there are others who disagree. These latter do not indeed hold to 

 the crude doctrine of " use inheritance " as propounded by Lamarck, 

 but maintain that a changed environment will after a certain 

 number of generations impress itself on the germ cells and bring 

 forward a certain amount of experimental evidence to support 

 this. One of the best known of these is Tower's work on breeding 

 beetles of the genus Leptinotarsa under different environmental 

 conditions, and noting the effect upon the offspring. The subject 

 is too wide to be pursued further, but is one that has received a 

 great deal of attention, and much of the discussion it involved has 

 been concerning the use of words rather than the import of the 

 evidence considered. 



All these works, especially that of Darwin, caused a great deal 

 of attention to be paid to these questions, and, in particular, led to 

 a large number of investigations of the problems of heredity along 

 different lines. 



One of the most striking papers was that of Mendel, which was 

 published in 1866, and was not a result of previous activities. It 

 was, however, entirely overlooked, and its significance not appre- 

 ciated, owing to the discussion aroused by the " Origin of Species." 

 The paper was not heard of until it was rediscovered independently, 

 and practically simultaneously by three separate workers, Correns, 

 Tschermak, and de Vries, in different countries early in 1900. 



Johann Mendel read his two papers to the Natural History 

 Society of Brunn, where he was a priest, and soon after, unfortu- 

 nately for his scientific work, he was made Pralat of the monastery 

 of Brunn. Let us examine some of his actual experiments. He 

 crossed various varieties of peas, differing in certain points, and 

 observed numerically the result of breeding from these crosses. 

 Seven pairs of characters were investigated, but it is only necessary 

 to consider two here. Firstly, the seeds of different plants were 

 found to be more or less round and smooth, or irregular and deeply 

 wrinkled. Secondly, the seeds were noticed to be either yellow 

 (or orange) or green. In both cases the result was the same as that 

 in the following account where only the characters, smooth and 

 wrinkled, are considered, and it would still be accurate if yellow were 

 substituted for smooth, and green for wrinkled. 



2 H 



