66 DIFFERENTIATION [ch. viii 



differentiation was given by Guppy (11), whose concept it was that 

 in the far back days of damper and more uniform climate most of 

 what are now the large (or widespread, or both) families were 

 formed, each at one stroke by well-marked mutations, and they 

 then slowly began to grow in size by further mutations. As time 

 went on, and the earth perhaps became drier on the whole, the 

 variety of climate would increase, and mutations perhaps be 

 more rapid, but their "size" is supposed to have become less, so 

 that fewer great divisions, like for example the Monocotyledons, 

 tended to appear. As differentiation went on in the climates, so 

 it went on in the living forms. This does not mean that they were 

 necessarily formed in adaptation to the climates but rather 

 perhaps that the climatic change gave the stimulus which resulted 

 in further mutations. Mutations might be of any rank, from 

 variety up to division, so that any difference might appear at one 

 stroke. If the newly formed plant could pass through the sieve 

 of natural selection, and escape the dangers that threatened its 

 very existence when it first began, it might then begin to spread, 

 and once established in several places it would be, comparatively 

 speaking, safe. As the original species thus survived as well as 

 the offspring, the family must necessarily increase in number in 

 such a way that when plotted by their numbers of species, its 

 genera would form the "hollow" curve. It is quite possible 

 that after a certain lapse of time a species 77iust die out (43), 

 and it is still more possible that it may change into another by 

 some simultaneous mutation. We have seen a small instance of 

 simultaneous mutation in the sudden loss of smell that happened 

 to all the plants of musk some years ago, and may perhaps see 

 the results of series of such mutations in the consecutive species 

 of Stratiotes described by Miss Chandler (3) and other such 

 series. 



While under natural selection new forms only arise as the 

 result of improvements in adaptation, under differentiation they 

 arise because evolution must go on, at any rate whenever the 

 needful stimuli, or conditions, are present, as we have seen in the 

 case of the Podostemaceae (p. 20). Under natural selection the 

 small variety becomes a larger one, and so on. It seems to the 

 writer, as it did to Dr Guppy, that in trying to make evolution 

 work in this way, people have been trying to work it backwards, 

 and it is with the object of showing the necessity of proper 

 revision of the current view that the present book is written. 

 A number of more or less crucial test cases are given below, all of 



