72 DIFFERENTIATION [ch. viii 



appears in the group under consideration. Ruminate endosperm 

 being characteristic of all Annonaceae, and of none of the allied 

 Magnoliaceae, becomes very important in regard to these two 

 families, while in the palms, etc., it may characterise some only 

 of the species of a genus. Upon the theory of differentiation this, 

 of course, simply means that in the one case the ancestor that 

 showed it was the ancestor of a whole family, in the other only 

 of a few species. Any of those characters which we usually con- 

 sider as especially "family" characters may appear at any stage 

 from family down to species, but on the whole are more common 

 as one goes upwards in a family from the species. 



One thing that is always brought up as an argument against 

 those who object to the explanation of evolution by natural selec- 

 tion is that the fossil records show many extinct genera, of 

 families still existing. The theory of natural selection, based upon 

 adaptation, with its prompt killing out of less-adapted ancestors, 

 accounts easily for this, while differentiation, which supposes the 

 ancestors to live on together with their descendants, cannot do so. 

 But one is apt to forget that the explanations of the facts of 

 palaeobotany have for many years been such as could be made to 

 jfit with the all-powerful theory of selection. One is reminded of 

 the defence of phrenology in The Professor at the Breakfast Table. 

 There are a number of things that must be taken into considera- 

 tion before one can fully explain the fossil records. 



In the first place, it seems not impossible, as Small has shown 

 (43), that there may be a definite limit to the life of species and 

 genera. In his summary he says: "From this the important de- 

 duction can be made that species die a normal death, presumably 

 from the senescent sterility of old age, with, perhaps, a minor 

 part being played by the progressive restriction of survival condi- 

 tions for a senescent species . . . the species number in a genus is 

 shown to follow the series 



1-2-4-8-16-32-44-43-41-37-29-13-0. 



This gives 24 million years as the normal lifetime of an 

 ordinary genus." 



This is supported by such facts as those brought out by Miss 

 Chandler (3), who found in different recent horizons a whole 

 series of fossil species of Stratiotes, differing structurally from one 

 another, but with nothing to which one could possibly attribute 

 any adaptational value. The loss of smell by musk (p. 66) shows 

 that a whole species can undergo a simultaneous change ; a larger 



