INHERITANCE 307 



isolated aphid begins to bear, remove the young as fast as 

 found to the leaves of the second enclosed plant, leaving the 

 original aphid alone, for certain determination of the num- 

 ber of her progeny. Observe in the other cage the time of 

 beginning of reproduction on the part of the descendent 

 aphids. 



The record of this study may consist of diagrams illus- 

 trating the method used, and a statement of observations 

 made. 



II. THE OBSERVABLE RESULTS OF INHERITANCE. 



As bearing on the points just cited, we may note that 

 many facts indicate the uniformity of development, when 

 cell increase proceeds by regular mitotic division, and, on 

 the other hand, that marked changes result from cross fer- 

 tilization. This is, perhaps, most familiar to the horticul- 

 turist, who maintains his choice varieties of fruits by rigid 

 adherence to asexual methods of propagating them, (cut- 

 tings, layers, stolons, buds, etc.), well knowing that cross 

 fertilization would introduce new characters to modify (and 

 from his point of view, to deteriorate) his commercially 

 valuable strains. The breeder of domesticated animals has 

 not this advantage. He can increase his flocks only by 

 bisexual reproduction. Hence, he must isolate his pure 

 bred individuals in order to maintain the purity of his stock. 

 This is the key to the efficiency of isolation, as well in 

 nature, as in plant or animal industry. 



Pure breeds and hybrids. In nature, the individuals of a 

 species usually present great uniformity of appearance. 

 They "breed true." But in some wild species there are 

 diverse forms more or less constantly appearing. Some- 

 times group-differences are correlated with habitat, as in the 

 case of the spermophiles (ground squirrels) of our Pacific 

 slope, where nearly every valley has its own peculiar variety. 



