454 GENERAL RESULTS. CHAP. XII 



long lain dormant; for gardeners believe that the 

 production of double flowers and of fruit is thus in- 

 fluenced. Seeds, moreover, which were matured during 

 different seasons, will have been subjected during the 

 whole course of their development to different degrees 

 of heat and moisture. 



It was shown in the last chapter that pollen is often 

 carried by insects to a considerable distance from 

 plant to plant. Therefore one of the parents or 

 ancestors of our two plants of Digitalis may have been 

 crossed by a distant plant growing under somewhat 

 different conditions. Plants thus crossed often pro- 

 duce an unusually large number of seeds ; a striking 

 instance of this fact is afforded by the Bignonia, pre- 

 viously mentioned, which was fertilised by Fritz Miiller 

 with pollen from some adjoining plants and set hardly 

 any seed, but when fertilised with pollen from a dis- 

 tant plant, was highly fertile. Seedlings from a cross 

 of this kind grow with great vigour, and transmit their 

 vigour to their descendants. These, therefore, in the 

 struggle for life, will generally beat and exterminate 

 the seedlings from plants which have long grown near 

 together under the same conditions, and will thus tend 

 to spread. 



When two varieties which present well-marked 

 differences are crossed, their descendants in the later 

 generations differ greatly from one another in ex- 

 ternal characters ; and this is due to the augmentation 

 or obliteration of some of these characters, and to 

 the reappearance of former ones through reversion; 

 and so it will be, as we may feel almost sure, with any 

 slight differences in the constitution of their sexual 

 elements. Anyhow, my experiments indicate that 

 crossing plants which have been long subjected to 

 almost though not quite the same conditions, is the 



