DISCONTINUOUS VARIATION. 71 



has brought forward evidence that some races of man 

 show diminished fertility together. 



In order to obtain further evidence, the author * 

 made numerous observations on the effects of crossing 

 the colour varieties of the sea-urchins Spliccrecliinus 

 granularis and Strongylocentrotus lividus. With the 

 former organism the numbers of blastulae and of larvae 

 produced on crossing dissimilar colour varieties were 

 distinctlv smaller than for similar varieties. In the 



y 



most marked instance, the similar color varieties yielded 

 on an average 98.5 per cent, of blastulae, and 73 per 

 cent, of larvae, whilst the dissimilar yielded 68 per cent, 

 of blastulse and only 15.6 per cent, of larvae. More- 

 over these latter larvae were 4.5 per cent, smaller than 

 the others. In the case of Strongylocentrotus, however, 

 where the colour varieties are much less pronounced, 

 there was very little difference of fertility. 



There can be no doubt, therefore, that certain varie- 

 ties show a greater or less degree of mutual infertility, 

 though this is doubtless not nearly so marked, or of 

 such frequent occurrence, as in the case of species. 

 Whatever view be taken as to the cause of such infer- 

 tility, and its relation to divergent evolution, these in- 

 stances quoted have also an intrinsic value. They show 

 that just as the deviations from the average in respect 

 of morphological characters may form double humped 

 curves, so the deviations in respect of physiological 

 characters may show corresponding irregularities, 

 though of course it is impossible to measure them 

 exactly and construct their curves of variation. 



* Phil. Trans. 1898, B. p. 511. 



