FERTILISATION 201 



The importance of Mendel's discovery lies in the fact that it 

 forms the basis of a theory whereby variability can be discussed in 

 terms of the conjugating cells themselves, and not merely in terms of 

 the resulting zygotes. Moreover, it is a theory which has been found 

 to be applicable to a very wide class of facts ; and it differs from 

 other theories of heredity in that it stands the test of a true scientific 

 hypothesis in enabling one to predict phenomena which on no other 

 theory could be predicted. 1 There are reasons for supposing that sex 

 may be a Mendelian phenomenon ; that is to say, that the ova and 

 spermatozoa are themselves sexual entities prior to conjugation (see 

 p. 671). It still remains to be proved, however, that the principles 

 underlying Mendel's theory are applicable to all forms of inheritance. 2 



It has been mentioned that on Weismann's hypothesis every 

 gamete contains ids representing both its parents and all its 

 immediate ancestors. On the other hand, according to the Mendelian 

 theory, although all the essential characters of the organism are 

 represented in each germ-cell, the Mendelian characters, or allelo- 

 morphs as they are called, are each represented by paternal or 

 maternal ids only, and not by both, while the immediate ancestors 

 have no representation at all. It has been supposed that the 

 chromatin granules (which Weismann identified with the ids) are 

 the carriers of the Mendelian allelomorphs, and that when these 

 fuse together during the conjugation of the chromosomes which 

 precedes the process of reduction (see p. 125), there is an exchange 

 of allelomorphs between the chromosomes. 3 If this interpretation 

 is correct, it is simply a matter of chance whether an allelomorph 

 remains in the chromosome which originally contained it, or becomes 

 transferred to the other chromosome of the conjugating pair. And 

 since each of the two chromosomes passes into a different product of 

 cell division, the allelomorphs would become distributed in precisely 

 the kind of way that the Mendelian theory postulates. 4 



The Mendelian investigators have shown that by experimental 

 breeding it is apparently possible to superimpose certain characters 

 belonging originally to one kind of individual, upon different 

 characters belonging to another kind, thus creating new combina- 

 tions of characters. Thus it is claimed that by starting with two 

 individuals, each possessing two unit or allelomorphic characters, 

 which we may call A and X (associated together in one individual) 

 and B and Y (associated in the other), it is possible in two genera - 



1 Mai-shall, "The Categories of Biological Science," Mind, vol. xxix., (Jan. 

 1920. 



2 Cf. Darbishire, "Recent Advances in Animal Breeding," Royal Horti- 

 cultural Society's Report of the Conference on Uenetics, London, 1J>07 



3 For the evidence see Morgan, Io<: fit. 



4 Lock, lot; <-it. 



