10 NATURE SKETCHES IN TEMPERATE AMERICA 



from the two parents, and it is possible that neither parent 

 may manifest all the peculiarities which it transmits to 

 the offspring. For the parent is made up of two distinct 

 parts, its own body and the reproductive substance con- 

 tained wuthin that body, and the two may not be identical 

 in character." 



The checks to inheritance as far as we know them are: 

 firstly, circumstances hostile to the pecuhar characters in 

 question; secondly, conditions of life incessantly inducing fresh 

 variability; and, lastly, the crossing of distinct varieties during 

 some previous generation, together with reversion; that is, 

 the tendency in the child to resemble its grandparents, more 

 remote ancestors, or some distant member in a collateral line 

 instead of its immediate parents. 



A familiar example of reversion is the horns appearing in 

 the young of hornless cattle. The offspring may present inter- 

 mediate characters between those of the parents, which is 

 known as blended inheritance; or the offspring may be more 

 extreme than either parent, which is known as intensified 

 inheritance; or, lastly, the offspring may exhibit characters dif- 

 fering from those of either parent, which is known as hetero- 

 geneous inheritance. In the case of sterile offspring, such as the 

 mule, of course they are self-exterminating. Mendel brought 

 forth the principle that the hybrid produces germ-cells like 

 those of its parents in about equal numbers, and the character 

 of its offspring will be dependent on the chance way in which 

 these germ-cells are paired in fertilization. When the parental 

 characters are preserved in the hybrids, that is unaltered, it is 

 known as alternative inheritance. 



Castle affirms that size variation is apparently continuous 

 and its inheritance blending, while color variation is discon- 

 tinuous and its inheritance is governed by Mendel's law.' 



Brooks reminds us that in popular language specific stability 

 may be said to be due to inheritance and specific mutability 

 to variation. But in this connection these words have only a 

 loose meaning, in so far as they convey the impression that the 

 stability and mutability of species are antagonistic to each 

 other. These terms are unfortunate, for we have good ground 

 ' Popular Science Monthly, May, 1910, p. 426. 



