INBEEEDING, PARTHENOGENESIS, ASEXUAL REPRODUCTION 255 



in those Avith long styles. Experiments have proved that the dusting of 

 the stigma has the best results if pollen from the short-styled reaches 

 the stigma of the long-styled form, or if pollen from the long-styled 

 form reaches the stigma of the short-styled. Thus we have again 

 to deal with an arrangement for crossing, an adaptation to the 

 advantages of cross-fertilization, and we can in this case see the reason 

 why the pollen has a different effect upon the two stigmas : the pollen- 

 grains of the flowers with short style are larger than those of the 

 flowers with long style, and as the length of the pollen-tube that can 

 be sent out must depend upon the mass of protoplasm within the 

 pollen-grain, it follows that the smaller pollen-grains will send out too 

 short a tube to reach through the long style to the embryo-sac. In 

 addition to this there is a difference in the papilla of the stigmas, and 

 it is possible that these may form an obstacle to the penetrating of 

 pollen from a similar type. The process of selection which gives rise 

 to such arrangements as we find in Primulas may easily be imagined, 

 as soon as we are able to assume that cross-fertilization is more 

 advantageous than self-fertilization as regards progeny, that is, as 

 regards the continuance of the species. 



We have already seen that uninterrupted self-fertilization is 

 unknown among animals, but that it is not even very rare among 

 plants, and this emphatically corroborates our previous conclusion, 

 that the reason for which amphimixis was introduced as a normal 

 event in nature is not to be sought for in the necessity for a renewing 

 of life, or 'rejuvenation.' It cannot be a necessity, but only an 

 advantage, which can in certain circumstances be dispensed with. 



Although it is obvious enough that continued inbreeding in its 

 most extreme form, self-fertilization, does not imply an absolute 

 abandonment of amphimixis, the adherents of the rejuvenescence 

 theory have regarded the unfavourable consequences of pure inbreed- 

 ing as a confirmation of their assumption, according to which amphi- 

 mixis is indispensable to the continuance of the life of the species, 

 and it is therefore an important fact, if it can be proved, that con- 

 tinued self-fertilization can occur persistently, among plants at least, 

 and yet not cause any injurious results to the species. 



But how can this fact be understood from our point of view ? 

 How does it happen that crossing is striven after in so many different 

 ways and yet so often given up again, and continued self-fertilization 

 resorted to 1 



To this it may be answered, in the first place, that it is not, as 

 far as we can see, for internal reasons that persistent self-fertilization 

 becomes the rule ; there is no peculiar condition of the germ-plasm 



