1006 THE PHYSIOLOGY OF REPRODUCTION. 



the germ plasm is such that it may at times give rise not to fluctu- 

 ating variations, but to marked and permanent variations, and 

 these latter, if advantageous to the animal, are preserved by 

 natural selection. Such permanent variations are known as 

 mutations or "sports," and in consequence of their formation 

 and preservation the process of evolution may proceed much 

 more rapidly than was assumed to be the case in the original 

 form of Darwin's hypothesis. The contribution made to our 

 understanding of heredity by the work of Mendel and those who 

 have used his conceptions is most significant. By the Mendelian 

 law or Mendelian inheritance is meant in the first place the general 

 idea that characteristics handed down by inheritance from parents 

 to offspring may be treated as separate units. It is implied in thia 

 view that these unit characters are conveyed by definite substances 

 or structures in the germ plasm which, for convenience, have been 

 designated as ''determiners" or "unit-factors." Their nature 

 is, of course, unknown. In some cases parental characteris- 

 tics may apparently blend in the children, as, for example, in the 

 case of color, the mulatto being in this regard a blend of a white 

 and a black parent. In most cases, however, there is no blend- 

 ing, but an alternation of one or the other of a pair of contrasting 

 characteristics. As regards such a pair of alternating character- 

 istics Mendel found that one will be dominant, the other recessive, 

 whenever they are brought together. That is to say, if each parent 

 possesses one of such alternating characteristics, brown eyes and 

 blue eyes, for example, the children will all show the dominant 

 characteristic, in this case brown eyes, but the other characteristic 

 will be present in a recessive or concealed form. In the hybrids 

 possessing both characteristics the germ cells are so divided that 

 half of them possess the dominant alone and half the recessive 

 alone. This constitutes the law of the "purity of the germ cells" 

 or of the "segregation of the gametes." If two such hybrids breed 

 together it follows from the law of probabilities that in the offspring 

 three out of four will show the dominant characteristic and one the 

 recessive characteristic. Moreover, of those that show the domi- 

 nant characteristic two will be hybrids, containing also the recess- 

 ive, but one will be a pure dominant. This result may be under- 

 stood from the following formula, in which D and R represent 

 respectively the dominant and the recessive: 



D— R 



I 1 = 1 DD, 2D(R) and 1 RR. 

 D— R 



If two pure recessives or two pure dominants breed together, only 

 a recessive or a dominant, as the case may be, will be exhibited 

 in the offspring, and in this way pure characteristics may be 



