18 INVEKTEBKATA CHAP. 



the male parent as the female, and there must come a point when 

 the hereditary influence of the male asserts itself. This, Loeb 

 suggests, is when the foreign chromatin becomes dissolved and 

 spreads its influence through the cytoplasm. 



Now the common experience of breeders, as collected by Darwin, 

 bears witness to the beneficial effect on the vigour of the offspring 

 which is gained by crossing two parents of slightly different strains. 

 Their experience, moreover, is, that in what is termed inbreeding, 

 that is, when the male is nearly related to the female in blood, the 

 resulting offspring exhibit weakness of constitution. In the case of 

 the higher plants, which have both kinds of germ in the same 

 individual, self-fertilization produces in many cases similar results. 

 This can be explained if we imagine that in the normal multiplica- 

 tion of the cells in a developing organism, starting from the zygote 

 and leading through many cell-divisions to the formation of germ 

 cells, the wastage of the original chromatin is not quite made good. 



From experiments made on Protozoa we conclude that without 

 chromatin no assimilation or building up of fresh living material can 

 take place, and we can only imagine that the influence of chromatin 

 is exerted through the substances which it is constantly giving off 

 into the cytoplasm. Now if our assumption be just, the germ cells of 

 each generation should become more and more imperfect in their 

 chromatin equipment, and this imperfection should exhibit itself as 

 a diminution in vitality. Hence we conclude that the prime object 

 of conjugation is to maintain the vitality of the stock by adding 

 together two chromatins of slightly different kinds, which will 

 presumably not be deficient in the same places. On this view the 

 purpose of reproduction by unicellular germs would be to render it 

 possible for the two chromatins to be thoroughly mixed. 



Herbst (1900) has started an egg to develop by using hypertonic 

 solutions and valerianic acid, and then, when the nucleus had divided 

 into two, he has fertilized the bicellular organism with spermatozoa. 

 As a result, one of the two nuclei has conjugated with the sperm 

 nucleus, and an organism was produced, on one side of purely 

 maternal character and on the other side showing the paternal 

 influence. 



It has been asserted in contradiction to this that there are some 

 plants, such as the Pea, which normally fertilize themselves and yet 

 undergo no deterioration, and others like Hieracium, the hawkweed, 

 in which sexual reproduction has been entirely replaced by asexual. 

 But such isolated cases cannot be allowed .to weigh in the balance 

 against the great mass of evidence which tells in the contrary 

 direction. The deterioration due to inbreeding may be very slow 

 in showing itself, and a cross at long intervals may be sufficient to 

 restore vitality ; and he would be a rash man who would deny this 

 possibility in the case of any of the species which apparently undergo 

 continuous self-fertilization. 



Leaving now the question of the meaning of the sexual element 



