42 2 The Living Plant 



characters of the organism, but never any duphcate units. On 

 the other hand, the fertihzed egg-cell, and every cell of the body 

 subsequently arising therefrom, contains a duplicate or double 

 set, one from each parent, from which selection has to be made 

 during the development of the organism; and this double set pre- 

 vails until the new formation of the germ cells, each of which 

 in the ''reduction division" receives but a single set. 



Third, while each one of these newly-formed germ cells contains 

 a complete set of units, these are partly derived from one of the 

 parents, and partly from the other. Moreover, in any given germ 

 cell, the paternal and maternal units are mixed in the most com- 

 plicated manner, and, furthermore, hardly any two germ cells can 

 be found with the same combination. Consequently when the 

 unions of these germ cells in reproduction are left wholly to 

 chance, as Mendel's results prove that they are, then the most 

 diverse possible combinations of paternal and maternal charac- 

 ters must result, even among close-fertilized kinds such as many 

 plants are, while the complications are proportionally greater 

 among cross-fertilized beings, like mankind. We have thus the 

 explanation of the very familiar fact that no brothers or sisters 

 are ever found who exhibit the same combination of characters of 

 father and mother, — even the case of identical twins being no real 

 exception, since these are known to arise from the splitting of one 

 fertilized egg-cell. 



We have now brought our subject quite down to our own days, 

 which are distinguished by extreme activity in experiment. It is 

 not possible, however, to estimate as yet the value of the results 

 that seem to be accruing therefrom. We lack perspective, of 

 course; and moreover the conclusions have not yet received the 

 thorough critical testing which is essential to establish that 

 "impersonal validity," without which they cannot rank as 

 scientific knowledge. But I shall add here a sjmopsis of the 

 principal matters which seem to be crystallizing out from these 

 studies. 



