428 THE STUDY OF ANIMAL LIFE CHAP. 



advances have been made. (1) It has become clear that 

 heredity is not a force or power, but the relation of genetic 

 continuity between successive generations ; that inherit- 

 ance includes all that the organism is, or has, to start 

 with in virtue of its hereditary relation ; and that 

 development is the expression of the hereditary " nature ' 

 under the influence of appropriate " nurture." The idea 

 of the continuity of the germ-plasm, Avhich has been 

 already explained, has clarified the general concepts of 

 heredity and inheritance. (2) There has been an increas- 

 ing precision in the demonstration of the heritability of 

 particular characters, including subtle qualities like lon- 

 gevity and fecundity and immunity. Excepting sterility, 

 any kind of character that appears as an inborn feature 

 in an organism may be transmitted to the next generation. 



(3) It has been shown that there are numerous hereditary 

 characters which behave in a distinctive and independent 

 way in inheritance : they do not blend or intergrade, they 

 are infallibly present in a certain proportion of the off- 

 spring, they are either there or not there. Such char- 

 acters are called " unit-characters," and sometimes, at 

 least, they arise as mutations. In his book on Mendelism, 

 Prof. R. C. Punnett refers to a unit-character as follows : 



" Unit- characters are represented by definite factors in the gamete 

 which, in the process of heredity, behave as indivisible entities, 

 and are distributed according to a definite scheme. The factor 

 for this or that unit-character is either present in the gamete (i.e. 

 the germ- cell), or it is not present. It must be there in its entirety, 

 or be completely absent." 



(4) Besides the Mendelian mode of inheritance, there 

 appear to be others, especially what is called blending, 

 where the offspring is, as regards certain characteristics, 

 like a thoroughgoing mixture of the corresponding 

 paternal and maternal qualities. But many instances 

 of what used to be called blending and of what used 

 to be called reversion, and regarded as distinct modes 

 of inheritance, turn out to be interpretable as Men- 

 delian. (5) The result of much discussion and a few 

 striking experiments bearing on the transmissibility 



