156 TEST CASES [ch. xiii 



that all the offspring will carry a large proportion of the cha- 

 racters of the parent, and that therefore while offspring arising 

 near together will be most likely closely to resemble one another, 

 there is no reason why a close resemblance should not arise 

 with a wide geographical separation. 



It is rare to find a genus going far outside the limits of the 

 genus that may be looked upon as the linking genus (e.g. Draba 

 in Drabeae). When it does, one may imagine that in its " make-up " 

 there was included a greater suitability to conditions that may 

 be a barrier to the parent — it may be capable of growing in 

 warmer (or colder), wetter (or drier), or otherwise different 

 localities. 



The author is not attempting to set up this " parent and child " 

 theory as a universal rule, nor at present attempting to apply 

 it to zoology; but there is no doubt that it will apply very well 

 to most of the small families of plants, to a great number of the 

 larger families, to a great number of the subdivisions of families, 

 and to a great number of genera whose species behave as do those 

 genera that we have been dealing with. 



The theory of accumulation of small differences makes many 

 of these and other phenomena very difficult to understand. To 

 get two closely related genera or species so widely separated 

 geographically by aid of the selection of small differences would 

 be very difficult, for one would have to assume — if the differences 

 be regarded as adaptational — that the conditions in the two 

 places were very similar, though there is little evidence to that 

 effect, or that the genera once touched one another in their 

 distribution, and that there has been a vast amount of destruc- 

 tion. Not only so, but this destruction must have gone on through 

 every variety of conditions to which the genera must have been 

 adapted. There is some change and variety to be passed through 

 between Greece and California, for example, or between Persia 

 and Cape Colony, to take a couple of examples from the Lepideae. 



It is probable that cytological study will throw some light 

 upon this difficult problem and it is clear that what has been 

 said here is fully in favour of the theory of differentiation, 

 affording no support to that of natural selection. 



TEST CASE XXIX. VARIETY OF CHARACTER 

 WITH UNIFORM CONDITIONS 



We have seen (p. 18) that the Podostemaceae and Tristichaceae, 

 growing in the most uniform conditions that it is possible to 

 imagine, yet show a very great variety of character and of 

 structure. And not only so, but the characters are at times very 

 definitely divergent, such things showing as bilocular and unilo- 

 cular ovary, one stamen or two, many seeds or two to four, and 



