230 HISTORY OF BOTANY 



25 dwarf ovules by 25 dwarf pollen grains, and in both 

 cases the products of the resulting zygotes (homozygotes) 

 will breed true, for in the first 25 zygotes there is no 

 dwarfness, and in the second 25 there is no tallness. 

 The remaining 50 zygotes, however, have tallness and 

 dwarfness combined (heterozygotes), but, since tallness 

 is the dominant character, all the plants produced will 

 be tall. Thus on theoretical grounds we ought to get 

 25 true tails, 50 dominant tails, and 25 true dwarfs, which 

 is exactly what we do get from experiment. Further, 

 the 50 dominant tails when bred from must each give the 

 same results as were given by the original pair, and so on 

 for each successive generation. 



When plants with two or more alternative characters 

 were experimented with, Mendel found that, although 

 each pair of characters followed the law I have just 

 stated, " the inheritance of each pair was absolutely 

 independent of the other." 



How, you may next ask, will this affect our conception 

 of variation and the doctrine of evolution as a whole ? 

 That species vary greatly is well known to you, and you 

 have only to look at any Enghsh Flora to see recorded 

 often a large number of varieties under the species 

 headings, e.g. the willows, brambles, buttercups, and 

 roses. But if the number of " factors " or " unit char- 

 acters " is comparatively small, how is this great variation 

 to be accounted for ? Let us see. If there be only one 

 factor there can be only two possible forms, but if there 

 be ten, then there may be 2^^, or 1024 possible forms ; 

 where the heterozygous and homozygous forms differ in 

 appearance there will be three possible forms for each 

 factor, and if there be 20 such factors the possible number 

 of different individuals will be 3-^ or 3,486,784,401, 

 hence the range of individual variation may be enormous, 

 and, provided that the constitution of the gametes is 

 unchanged and the respective factors are clearly defined, 

 each variation concerned will be transmitted in obedience 



