98 PROBLEMS OF GENETICS 



depends on a simple dominant factor, because the big size of 

 the crosses may be a consequence of the cross and may depend on 

 other elements. 



Now no wild fowl known to us has these qualities. May we 

 suppose that some extinct wild species had them? If so, may 

 we again make the same supposition in all similar cases? To do 

 so is little gain, for we are left with the further problem, whence 

 did those lost wild species acquire those dominants? Supposi- 

 tions of this kind help no more than did the once famous 

 conjecture as to the origin of living things that perhaps they 

 came to earth on a meteorite. The unpacking of an original 

 complex, the loss of various elements, and the recombination of 

 pre-existing materials may all be invoked as sources of specific 

 diversity. Undoubtedly the range of possibilities thus opened 

 up is large. It will even cover an immense number of actual 

 examples which in practice pass as illustrations of specific dis- 

 tinction. The Indian Rock pigeon which has a blue rump 

 may quite reasonably be regarded as a geographically separated 

 recessive form of our own Columba lima, for as Staples-Browne 

 has shown the white rump of livid is due to a dominant factor. 

 The various degrees to which the leaves of Indian Cottons are 

 incised have, as Leake says, been freely used as a means of classi- 

 fication. The diversities thus caused are very remarkable, 

 and when taken together with diversities in habit, whether 

 sympodial or monopodial, the various combinations of points 

 of difference are sufficiently distinctive to justify any botanist 

 in making a considerable number of species by reference to them 

 alone. Nevertheless Leake's work goes far to prove that all of 

 these forms represent the re-combinations of a very small number 

 of factors. The classical example of Primula Sinensis and its 

 multiform races is in fact for a long way a true guide as to the 

 actual interrelations of the species which systematists have 

 made. That they did make them was due to no mistake in 

 judgment or in principle, but simply to the want of that ex- 

 tended knowledge of the physiological nature of the specific 

 cases which we now know to be a prime necessity. 



But will such analysis cover all or even most of the ordinary 



