PRINCIPLES OF HEREDITY 215 



merely an expression of that plasticity which evolution has im- 

 posed on all Uving protoplasm ? In other words, does it depend 

 upon some definite change in the architecture of the germ-cells 

 and is therefore transmitted from one generation to another ? 

 Or is it simply a matter of a nutritional responsive capacity of 

 the body-cells, an event for this generation only, and the outcome 

 of a passing influence of environmental combinations ? If it be 

 that, its manifestation through succeeding generations will 

 depend upon the continued presence of these same environmental 

 influences. But if it is not that, then its manifestation will be 

 independent of particular combinations of environmental in- 

 fluences, and throughout long ranges of geographical distribution, 

 and under diverse conditions, will exhibit its particular quahty, 

 whether it is size or any other. 



Now, it is known that there are species, the individuals 

 of which live under very different conditions, and yet preserve 

 an identity of specific characters. We may cite the case of a small 

 snail {Helix arbustorwn var alpestris) which is found hving above 

 the snow hne on the Alps and also in the marshes of Hoddesdon, 

 in Herts. Notwithstanding that these conditions are wholly 

 different, the individuals living in the one region are not distin- 

 guishable from those hving in the other. Or, we may show the 

 same thing by taking a converse case, namely, where distinct 

 forms within a species maintain their characteristic differences 

 though grown under the same conditions. The vernal whitlow 

 grass is a case of this sort. This species has a large number 

 (about 200) of forms which differ from each other in several 

 features. These forms breed true, both in their local stations 

 and when brought together and reared in botanical gardens. 

 Take, for instance, two of these forms, one with narrow leaves 

 and the other with a broader foliage. They have been cultivated 

 side by side, and yet each form retains its own peculiar features 

 in a quite imiform way. In Nature, on the geographic borders 

 of the limits of the different forms, intermediates are not known. 

 Hence, it is clear that some minor and greater differences of 

 character which mark the individuals of many species are of 

 fixed gametic origin, and though in some cases small in amount, 

 are yet quite distinct and unblendabl.e. Experiment alone can 

 decide for certainty whether these forms are really unblendable 

 when crossed. For the present, we have only the evidence of 

 the absence of intermediate forms on the geographic limits of 

 the overlapping groups. But still it is known that in other 

 species similar differences do not blend in inheritance. 



Then there is the wide range of comparative studies, where 

 in comparing the extremes of numerous variations of very 

 different kinds, we fail to find evidence that intermediate stages 



