226 MENDELISM 



by an entirely new set of ideas. And it may be 

 remarked in passing that the biologist of fifty years 

 ago and more was much nearer to our present line of 

 inquiry. 



We have seen enough to enable us to recognise 

 very clearly the vital importance of an understanding 

 of the constitution of the gametes in all questions of 

 heredity. There must exist in the gametes, in an 

 uncombined condition, those units which by their 

 combination in zygotic organisms lead to the appear- 

 ance of the characters which we can recognise. But 

 we have seen that, owing to the appearance of domi- 

 nance and other kindred phenomena, the visible external 

 characters of an organism are not a complete guide to 

 the nature of its gametes. It is only by careful 

 breeding that we can distinguish the heterozygote from 

 the pure dominant form to take the simplest possible 

 example of this difficulty. For this reason it has now 

 become the chief business of the student of heredity 

 to determine by experiment what combinations of 

 allelomorphs are present in the gametes of the indi- 

 viduals with which he is working. 



The behaviour of these allelomorphs has now been 

 disentangled in many cases of very considerable com- 

 plexity ; and all such cases as have been so far examined 

 hi detail have proved explicable in terms of a larger or 

 smaller number of allelomorphic pairs, all of which 

 obey Mendel's law with the single exception of those 

 cases in which coupling between the allelomorphs of 

 different pairs introduces a slight further complication. 



