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PROF. ERNEST W. MACBRIDE, M.A., F.R.S., ON 



from which all life was derived may have contained the factors 

 for all the qualities of which a Shakespeare might boast himself, 

 and that these qualities were prevented from exhibiting them- 

 selves merely by the presence of inhibiting factors which were 

 gradually dropped as time went on. 



There is an extreme left wing of the Mendelians, however, 

 who go further than this, and deny altogether the occurrence 

 of mutations. 



De Vries had maintained the view that every natural species 

 consists of several, sometimes of many, " elementary species," 

 i.e., of forms producing germs of different hereditary poten- 

 tialities, which in nature are continually intercrossing, and so 

 producing much of the variation which is observed in natural 

 species. The effect of mating selected pairs is, according to 

 De Vries, merely to purify gradually the selected stock and finally 

 to arrive at a race consisting of only one elementary species. 

 "When this goal has been reached, according to most Mendelians, 

 no further selection will have any effect in changing the 

 character of the stock. The difference between what we may 

 call a natural species and an elementary species, is that, speaking 

 broadly, two natural species either refuse to cross with one 

 another at all, or if they do cross will produce sterile offspring, 

 whilst two elementary species cross freely and produce fertile 

 offspring. 



We may now briefly review the situation at which we have 

 arrived, if the position taken up by Mendelians is a sound one. 

 We find then that there are practically an infinite number of 

 elementary species of animals and plants in the world, each 

 with its distinct definite and unalterable hereditary potentiality. 

 Groups of these are capable of crossing with each other and 

 constitute those populations known as species to the naturalists. 

 In this way continually new combinations of characters are 

 produced, from which, however, the characters of the original 

 elementary species are always tending to segregate out. The 

 process resembles exactly the dealing out of hands of cards from 

 a pack, which is being continually reshuffled. Some Mendelians 

 maintain that an infinite number of distinct hereditary poten- 

 tialities have existed from the beginning of life, and that new 

 forms can only arise and have only arisen by new combinations of 

 these potentialities (Lotsy). Others are willing to admit that 

 mutations, ie., changes in the hereditary potentiality, may have 

 taken place ; but these changes have always consisted in the 

 dropping of a factor, and in thus producing a form which, 

 compared to the original form, may be regarded as a cripple. 



