CHAPTER XIX 

 OTHER THEORIES OF SPECIES-FORMING 



THEORIES AUXILIARY TO NATURAL SELECTION 



The post-Darwinian causo-mechanical theories fall quite naturally 

 into two categories: those that were devised by Darwinians to bolster 

 up natural selection and to free it of some of its most obvious objec- 

 tions, while retaining the essential features of the principle; and those 

 that were meant to be substitutes for and therefore quite opposed to 

 natural selection. The former theories have been classed as auxiliary, 

 and the latter as alternative theories to natural selection. 



The several theories of Weismann will be dealt with first as the 

 most important of the purely auxiliary theories. "Panmixia" is 

 designed to explain, without recourse to Lamarckism and in harmony 

 with natural selection, the degeneration or atrophy of organs which 

 seemed to be inadequately explained by Darwin. "Germinal Selec- 

 tion" is supposed to explain the initial stages of adaptations and 

 allied phenomena, and thus to aid natural selection at one of its 



weakest points. 



weismann's theory of panmixia 



The following statement of "panmixia, " as given by S. Herbert, is 

 concise and to the point: 



"Cessation of selection as a cause of atrophy was first proposed 

 by Romanes. Later on, Weismann, whilst examining the validity of 

 the principle of use-inheritance, adopted the same idea, called by him 

 'panmixia,' in order to account for the dwindling and disappearance 

 of useless organs without having recourse to the Lamarckian factors. 

 If natural selection leads to the mating of select types, so that those 

 below a certain standard are prevented from propagating, it follows 

 that, with the cessation of selection, a general crossing of all types, 

 including the inferior ones, must take place, and thus lower the average 

 quahty of the whole stock. Weismann explained in this manner, for 

 instance, the prevalence of short-sightedness among civilized people. 

 The individuals with defective eyesight not being weeded out in 

 modem society, the sharpness of the eyesight of the population sinks 

 gradually. The same would apply to the deterioration of the teeth 



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