562 SCIENCE PROGRESS. 



with totally black forms. In some parts of this district, 

 however, as in Barmen and Crefeld, the typical doublc- 

 dayaria is said to be already the commonest form (8). But 

 there is no doubt both that the totally black form existed 

 at an early stage in the transformation side by side with the 

 light one, and that in localities where both occur the same 

 is still the case. The course has not been that the insects 

 of each successive district have become niore and more 

 tinged with black, till they culminated in double day aria, 

 but rather that doitbledayaria, or less often one of the inter- 

 mediate forms, spread into, or at least appeared in the area, 

 and either co-exists with the type or has replaced it. 



There is great need for experimental and quantitative 

 evidence as to the inheritance in crossed and thorough- 

 bred strains, and the subject is probably well suited to such 

 work. Nevertheless the results of the few experiments 

 that have been made are in accordance with the expectation 

 based on the simultaneous occurrence of the forms in 

 nature, that there is imperfect blending between type and 

 variety, for Steinert (9) raised from a typical black wild 

 female found at Dresden a brood consisting of seventy-five 

 bettilaria and ninety doubledayaria. Two of the betiilaria 

 were slightly darker than the normal form. 



Similar changes are said to be taking place in several 

 other British species of Geometrae, but the evidence seems 

 to be less clear and less extensive than it is in the case of 

 betiilaria. 



Here, then, we have the means not merely of studying 

 the relations of variety to type, but also of actually wit- 

 nessing a case of evolutive change. If some one will make 

 it his business to collect samples of betiilaria from various 

 districts he will certainly make a valuable contribution to 

 the science of evolution. All that is needed is that wild 

 samples should be taken at random, fairly illustrative of the 

 population of each district, and sorted into classes, (i) normal 

 betiilaria, (2) light intermediates, (3) intermediates, (4) dark 

 intermediates, (5) typical doitbledayaria. A simple way by 

 which something might be done would be to arrange year 

 by year for the exposure of a few fresh females reared in 



