736 A MANUAL OF VETERINARY PHYSIOLOGY 



some of the difficulties and complexities of the new doctrine ; their 

 consideration is beyond the scope of this work, and they require 

 expert handling. 



Evolution. — It is natural that Mendelism should be searched for 

 an explanation of the evolution of the domestic from the original 

 wild species. It is suggested by those to whose opinion great 

 weight is attached* that the addition of factors, the subtraction, or 

 the interpolation of factors will explain the little which is clearly 

 known of evolution. For instance, there appears to be no doubt 

 that the present sweet-pea in its many varieties arose from the wild 

 sweet-pea introduced into this country a little more than two hundred 

 years ago.f In this case the numerous present-day varieties have 

 arisen, as is supposed, by the subtraction of factors. All the 

 necessary elements for the production of the many civilised varieties 

 are believed to have existed in the wild plant, but one by one those 

 inimical to their production have been eliminated. The Mendelist 

 does not reject the influence of variation, but he attaches to it a far 

 stricter interpretation ; he does not believe that variation need 

 necessarily be small in amount, nor require ages to become adapted, 

 for it can be shown that marked variations, known as ' sports,' may 

 appear in a single generation. He further recognises two forms of 

 variation : the one which may appear quite suddenly, and is trans- 

 missible by the gametes, since it is due to the existence of ' factors ' ; and 

 the other which is acquired in consequence of the conditions of life 

 under which the organism is living. The first has been called mutation, 

 and has already been referred to ; the second is somatic fluctuation, 

 and, not being represented in the gametes, is consequently not trans- 

 missible. The process of evolution depends upon mutations, and 

 natural selection determines whether their continued production 

 shall be maintained or rejected. It is stated by PunnettJ that a 

 rare ' sport ' with 5 per cent, selection in its favour will replace the 

 normal in a few hundred generations. This being so, evolution 

 generally demands a far shorter time for its production than has 

 generally been supposed, and no better evidence in this respect can 

 be brought forward than the improvement within recent generations 

 of the breeds of domesticated animals, based on artificial selection. § 



Mendelism is still in its infancy, and its application to the breeding 

 of the larger stock, excluding poultry, has only been undertaken to 

 the extent recorded at p. 735. There are difficulties in obtaining 

 in animals the results readily effected in plants. One of these is 

 that the sexes have to be provided by two distinct physiological 

 units which may differ widely from each other, whereas in those 

 plants, which are produced by self -fertilisation the constitution of 

 the male and female elements is identical. 



The laws of inheritance of such unit characters as coat and eye 

 colour are known with considerable precision, but, as pointed out 

 by Bruce, || the laws of inheritance of such unit characters as size, 



* ' Mendelism,' R. C. Punnett, M.A., third edition, 191 1. 



f Op. cit. % Op. cit. 



§ In the matter of the race-horse, certain exact data are available for a 

 considerable period, and the bearing of Mendelian inheritance on this 

 animal has formed the subject of special study. See ' The Principles of 

 Heredity applied to the Race-horse,' by J. B. Robertson, M.R.C.V.S., 1910. 

 The writer takes this opportunity of acknowledging his indebtedness to 

 Mr. Robertson's work, and his communications thereon. 



|| ' Mendelism and its Application to Stock-Breeding,' by A. B. Bruce, 

 M.A., Journal of the Board of Agriculture, 1910. 



