200 



inevitably bad is shown by the fact that there exist 

 some animals and a number of plants in which self- 

 fertUisation — the most rigid form of inbreeding pos- 

 sible — is the invariable rule. In brief, it may be said 

 that our understanding of Mendelism has made it 

 clear that inbreeding is only bad when hidden harm- 

 ful factors exist in the stock, and that it is harmless 



RED 



WHITE 



DISCOVERY 



Two quite distinct processes are involved in 



the 



fertilisatiorj of egg by sperm which occurs in the 

 ordinary sexual reproduction of man and most animals. 

 First there is the fusion of two separate nuclei, two 

 single sfts of chromosomes, and the consequent 

 shuffling and recombination of the hereditary factors. 

 In the second place, there is what is usually called 

 aclivatioi — the starting-off of the egg upon its career 

 of grow:h and development. In most species, if 

 fertilisatbn does not take place, the egg remains inert, 

 and sooaer or later perishes. However, in some 

 animals, such as Aphids (plant-lice) and water-fleas, 

 the egg is capable of developing without this stimulus. 

 Such foms are called parthenogenetic. The bee is of 

 especial interest, since the males, or drones, are father- 

 less, produced parthenogeneticaUv, while the queens 

 and wo'kers all arise from fertilised eggs. In the 

 course of evolution, a change must have come about 

 so that activation is no longer carried out by the 

 sperm, but by some other means. What in these 



^^i> 



Chrtmosomes («) of 



norual female Dro- 



sophila . 



Reproductd from " Mendelism,' 



Fig. 3. 

 {b) Of normal male. 





(c) Of au XXY 

 female individual. 



by Prof. R. G. Punnell, F.R.S., by permission 



Fig. 2.— sex-linked inheritakce of white eye in DROSOPHILA. 



Females on left, males on right. The sex-chromosome behaviour is represented 

 diagrammatically ; the chromosome bearing the factor for red eye is represented in 



black, that bearing the factor for white eye in outline. 



Reproduced from " Mendelism." by Prof. R. G. Punmlt, F.R.S., by permission of the 



Author and of Macmillan & Co., Ltd. 



and even good when the stock's hereditary constitu- 

 tion is a good one. The ecclesiastical prohibition of 

 the marriage of near relatives is thus seen to be only 

 conditionally justified on biological grounds.' 



• The stimulating effect which often follows a cross between 

 markedly different stocks, while also explicable in terms of 

 factors, is due to rather complex causes. The reader is re- 

 ferred to the book by East and Jones. 



of the Author and of Macmillan & Co., Ltd. 



animals has happened naturally, has been accom- 

 plished artificially for others. In many creatures, 

 such as sea-urchins and starfish, marine worms, 

 moUus:s, and even frogs, it has been found possible 

 to make the egg develop without sperm. In sea- 

 urchins the best method is immersion in certain 

 chemicals ; in starfish it is heat or shaking ; in frogs 

 it is piicking with an extremely fine glass needle which 

 has been dipped in blood. The result is the same — 

 that fatherless individuals are produced by man's 

 intervention. Some of the fatherless frogs have been 

 raised in the ordinary way, and are apparently 

 healthy in every respect. This shows us that the 

 sperm normally performs two functions : it contri- 

 butes a quota of hereditary factors from the father 

 to the developing embryo ; and it activates the egg, 

 probably by chemical means, to start its career of 

 growth and division. This artificial parthenogenesis 

 has so far only been tried upon animals which lay 

 their eggs into the water before fertilisation ; there 

 can be no doubt, however, that it is theoretically 

 possible in other forms, and that it would be only a 

 matter of surmounting technical difficulties (although 



