President's Aeldress. 151 



That variation would provide material for both the length- 

 ening and reduction will be at once admitted, but it is of 

 course impossible to estimate at what rate the variations 

 would appear. The range of variation around a mean is 

 said sometimes to be over 20 per cent., with a continuous 

 addition of 10 per cent. At this rate the symphysis might 

 be increased from 75 mm. (the length in Moerithermm) to 

 nearly 200 mm. (half the length of the symphysis in 

 Pcdceomastodon) in ten generations. This is assuming that 

 there was no swamping, no reversion towards mediocrity, 

 and that the 10 per cent, gained by each generation would 

 be handed on undiminished to the next. 



Provided the lengthening of the symphysis and of the 

 trunk (which doubtless increased pari pccssu with the 

 mandible) counted in the struggle for existence, even if 

 there was only an increase of 1 or 2 per cent, in each 

 generation, the difficulty would obviously not be the want 

 of material with which to effect the elongation. Likewise, 

 when new conditions favoured more complex modifications, 

 viz., the reduction of the mandible to a mere spout-like 

 process, without a corresponding reduction of the trunk, 

 abundant variations would still be available. 



The difficulty in effecting these remarkable modifications 

 would not be a want of suitable variations or of selection on the 

 part of the environment. The difficulty consists in accounting 

 for the useful variations being preserved ; in explaining how 

 the progress achieved in one generation was handed on, even 

 in a greatly diminished form, to the next. Darwin fully 

 realised this difficulty, for he firmly believed in the principle 

 of reversion ; but, as far as I can gather, he assumed it could, 

 in most cases, be overcome by isolation, by preferential 

 mating, or by the transmission of acquired characters, and 

 other subsidiary aids of a Lamarckian kind. 



Romanes, as already pointed out, argued strenuously in 

 favour of physiological selection, and firmly believed that 

 proof would eventually be obtained in support of his assump- 

 tion that similarly modified varieties were frequently more 

 fertile with each other than with the parent species, or with 

 varieties modified in a somewhat different direction. 

 VOL. XV, M 



