Influence of External Conditions on Tricotyly. 451 



becomes a double one. To these is added a third factor, 

 namely the union of the two germ cells, which may itself 

 be influenced by outward conditions in different ways. 



I have gone into the questions only so far as was 

 necessary for the choice of the conditions of my experi- 

 ments. The course of these has, as a rule, been very 

 regular. The results of selection seldom have been dis- 

 turbed, to any great extent, by other influences. Some- 

 times, however, this did obviously occur ; and it is exactly 

 experiments of this kind which justify us in ascribing 

 an important part to external conditions in the determi- 

 nation of the proportion of tricotylous seedlings. But 

 when, from these facts, we proceed to analyze this in- 

 fluence, we do not succeed in making experiments in 

 which the same influences have the same results. 



Two instances will suffice. Under very peculiar con- 

 ditions which affected the whole growth of the plant, 

 the hereditary capacity of my race of Ainarantus spc- 

 ciosus suddenly rose, in 1891, from 4.5 to 13%, with- 

 out, however, maintaining that proportion even under 

 selection (p. 407). On the other hand, in the summer 

 of 1896 the whole culture of SUcnc inflata underwent 

 considerable deterioration as the result of unfavorable 

 conditions, the mean of all values falling from 53 to 

 32%. One year may obviously affect the plants quite 

 differently from another, even when the treatment to 

 which they are subjected is as uniform as possible. The 

 effect is perhaps produced in the early stages of the plant, 

 perhaps also in the development of the sexual cells and 

 at the time of fertilization. Abundant starting-points 

 for further investigations may here be found. 



As a rule the likelihood of obtaining anomalies in- 

 creases with the vigor of the seeds which produce them 



