204 COOK 



average, and those in which the parental diversities are preserved 

 by alternative expression. Nor is it to be supposed that there are 

 not conditions intermediate between scalar, polar and interme- 

 diate inheritance. In some characters there may be a tendency 

 to prefer the extremes of the series, and in others the mean, or 

 some other maximum point. Such relations have been inves- 

 tigated at length by Gallon and other statistical biologists. 



Scalar inheritance is in some respects intermediate between 

 indiscriminate inheritance and polar inheritance. The parental 

 diversities become established as the poles of the series, but the 

 polarity is not so definite as when the parental differences are 

 fully preserved. 



Scalar inheritance is commonly manifested in crosses between 

 selected varieties of domesticated plants and animals. It was 

 formerl}^ taken for granted that similar series of intergradations 

 were often produced in nature by the hybridizing of species, 

 but the evidence has failed to convince the late G. C. Churchill, 

 an English botanist who appears to have given this particular 

 point ver}'- extensive and discriminating study. ^ 



The fact seemed to be established that in the great majority 

 of cases, at least, the hybrids between adjacent species of gen- 

 tians and other alpine plants were limited to one or two definitely 

 intermediate forms, without series of connecting links or inter- 

 gradations (^(?<^£'r^>"rtr;/^5/t)r;;/^;/). The ability to form interme- 

 diate sterile hybrids would not really intermingle the species or 

 interfere with their continued differentiation. Less differentiated 

 species with separate geographical ranges might form inter- 

 graded crosses like selected varieties of the same species, but 

 it is not to be supposed that species which freely interbreed and 

 intergrade, could become differentiated while occupying the 

 same geographical distribution. Whenever two related, but 

 definitely distinguishable species occupy the same area we may 

 expect to find that they have some other form of segregation, 

 such as differences of time of flowering, or inability of the germ- 

 cells to effect normal conjugations. 



Descent zuith Presei'vation of Differences by Polar Inheri- 

 tance. — Polar inheritance is a general type of descent which in- 



' Churchill, G. C, 1906. Quotations in Obituary Notice, Kew Bulletin, No. 

 9, 1906, 3S4. 



