248 EVOLUTION, GENETICS, AND GENETICS 



true; the yellows when inbred showed themselves to be pure domi- 

 nants or "yellows" and dominant-recessives — i.e., splitting up again 

 into yellows and whites in the usual proportion. More intricate 

 experiments confirmed this general result. 



It must be noted, however, that Coutagne has made much more 

 elaborate experiments with different results, which in many cases can- 

 not be interpreted on the Mendelian theory. Thus he found (1) that 

 the hybrid forms were sometimes blends of the parents and different 

 from both; (2) that in other cases the brood included some like one 

 parent in a particular character, some like the other parent, and some 

 intermediate; and (3) that in other cases the individuals showed no 

 fusion of characters, but resembled one or other parent. It is likely 

 that the discrepancy may be explained as due to considerable diversity 

 of origin in the domesticated races of silkworm, so that, while they 

 breed true when left to themselves, a disturbance of the usual routine 

 leads to the liberation of latent characters. 



Lina lapponica. — Miss McCracken has made a fine study of the 

 hereditary relations in this Californian beetle, which occurs in two 

 types, spotted (dominant) and black (recessive). They are always 

 crossing in natural conditions, but there are no intermediates, and it 

 is easy by isolation to rear a "pure" spotted race and a "pure" black 

 race. When spotted forms are paired they may produce only spotted 

 progeny — a case of extracted dominants. In other cases, however, 

 they yield spotted and black forms (1,021 spotted, 345 black), i.e., in 

 the Mendelian proportion of 3 : 1 — a case of dominant-recessives inbred. 



Snails. — Lang paired "pure" five-banded forms of the common 

 or garden snail, Helix hortensis, with bandless forms from bandless 

 colonies. The young of the first generation were all bandless, the 

 banded character being recessive. When these were paired the off- 

 spring were bandless and banded in the Mendelian ratio, 3:1. Fur- 

 ther experiments confirmed this, not only as regards bands, but also 

 as regards colour (yellow or red), size, and the form of the umbilicus. 

 It may be said, therefore, that common snails {Helix hortensis and Helix 

 nemoralis) illustrate Mendelian inheritance. 



Poultry. — Numerous breeding experiments with poultry have 

 been made by Bateson, Bateson and Punnett, Hurst, Davenport, and 

 others, many of which show Mendelian phenomena with great clear- 

 ness, while others are strangely conflicting. One of the reasons for the 

 complicated results is evidently to be found in the difficulty of securing 

 thoroughly "pure" breeds, for many that breed true as long as they 



