GENERATION AND DEVELOPMENT 737 



shape, fertility, vigour, endurance, are still unknown. These must 

 be ascertained if exactitude is to replace the haphazard method of 

 breeding at present in force. The whole trend of the work on 

 heredity from the time of Darwin onwards has shown that it has a 

 physiological basis, and this, in the hands of the breeder, even when 

 employed in the dark, as hitherto, has yielded results of the highest 

 economic importance. It is not too much to hope that the great 

 precision given to heredity by Mendelism will in time replace the 

 policy of ' hit or miss ' by something approaching exactitude. 

 Already it has explained, through the simple term 'recessive,' the 

 previously incomprehensible fact that a character may ' skip a 

 generation ' ; it also offers an explanation of reversion * and atavism,^ 



Telegony is the supposed influence of a male by whom a female 

 has previously conceived, on her subsequent offspring by another 

 male. For generations this supposed influence was the dread of 

 breeders. It possessed no physiological explanation, and was 

 finally conclusively shown by Cossar Ewart to be without a shadow 

 of foundation. 



Heredity in Disease. — Of the influence of heredity in disease there 

 is no possibility for doubt. Arrested development of the fingers 

 in man, night-blindness, colour-blindness, haemophilia, and a few 

 other conditions, have been shown from constructed pedigrees to 

 follow, or closely approximate to, the laws of Mendelian inheritance. 

 So far as the hereditary diseases of animals are concerned, their 

 Mendelian examination has barely begun, and in the very nature 

 of things must take a long time to accomplish. Robertson has 

 shown, from his inquiry into ' Roaring and Ruptured Bloodvessels 

 in Race-horses, 'J the possibilities awaiting patient investigations. 

 On the question of the inheritance of disease considerable caution is 

 necessary. There are few veterinary practitioners who, with ex- 

 perience of stock-breeding, are not impressed by the hereditary 

 nature of such diseases as bone spavin, ring-bone, side-bone, ' shiver- 

 ing,' stringhalt, cataract, and navicular disease. 



It may be that our ideas concerning the actual nature of the 

 transmitted characters which lead to some, if not all, the above 

 conditions will have to be modified. Chemical composition, or even 

 molecular arrangement, may ultimately turn out to be the deter- 

 mining factor. No biologist would, for instance, assert that bone 

 spavin was represented by a unit character, but he would accept 

 the view that some particular condition of hock articular cartilage 

 led to bone spavin. He would also admit that this specific carti- 

 laginous condition might well be represented by a determining unit 

 character, and that this character passed from parent to offspring. 

 In these circumstances it is conceivable that bone spavin due to a 

 defect or modification in the germ plasm would be heritable. On the 

 other hand, if arising from sprain or trauma, its heredity would be 

 absent, for somatic acquirements are not transmissible (Robertson). § 



Robertson's investigations have shown that roaring, side-bone, 

 ' shivering,' and a tendency to ruptured bloodvessel, follow the laws 

 of Mendelian inheritance. 



* A return to some type of ancestral character, as when a black and 

 a white rabbit produce a grey, which is the colour of the wild form. 

 Chestnut is a reversionary character in the horse (Robertson). 



f 'Throwing back,' or atavism, is the appearance of ancestral charac- 

 teristics — as, for example, the extra digit sometimes found in the horse. 



X Op. cit. § Communicated. 



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