190 GENERAL DISCUSSION [ch. xiv 



that these later genera and species will be of much less wide 

 distribution than the earlier. 



The conceptions thus put forward have several possibly even 

 unexpected bearings. If new species and genera can thus arise in 

 widely separated places, though related, there seems no reason 

 why the same character, produced of course by some particular 

 arrangement of genes or chromosomes, should not at times arise 

 from ancestors in which it did not itself occur, i.e. should arise 

 polyphyletically, or from different ancestors. One may even 

 imagine more than one character arising in this way, so as to 

 form, though probably only with great rarity, a polyphyletic 

 genus. In some such way as this one may imagine the case of one 

 genus coming through another, as suggested by Bower in the 

 ferns (2). One must remember, too, that what look like species 

 of the same genus and closely allied, need not necessarily be such, 

 and one must compare their chromosome numbers. It is even 

 possible that originally separate types may converge until they 

 may be able to become cross-fertilised. 



The sudden appearance of similar mutations at widely separated 

 places may be easily accounted for by a similar construction in 

 the chromosomes of their ancestors, which might give rise to 

 similar mutations. There is no definite reason that one can see — 

 though, of course, this is unfamiliar ground to the writer — why 

 the same genie distribution should not appear in two new species 

 formed from one genus, thus giving rise to a new genus of two 

 species, and possibly even discontinuous in distribution. 



Finally, a very strong argument in favour of differentiation, 

 just as with Age and Area, is that by its aid one may make a 

 great many predictions as to what will be found to occur, and 

 find that these predictions are borne out by the facts. A number 

 of such are to be found in many of the test cases given in 

 chaps, x-xiii, and others may be found elsewhere. Now upon 

 the theory of natural selection it is as a rule impossible to make 

 any predictions at all, and when, as for example in several test 

 cases, one may venture a prediction, this is found to be opposed 

 to that made upon the theory of differentiation, and is not borne 

 out by the facts, which always favour the latter theory. This seems 

 to the writer to be a very strong argument in favour of differentia- 

 tion or divergent mutation. At first, owing to the fact that one 

 has to think, so to speak, in the reverse direction from that to 

 which one has been accustomed (i.e. from family to variety, not 

 from variety to family), it is not always easy, but one soon gets 

 into the new direction of thought. 



