30 CONTACTS WITH DARWINISM II [ch. iii 



meant considerable age, small area small (each set of plants com- 

 pared being taken from the same circle of affinity). 



And also, one must always remember that the distribution of 

 plants is very largely controlled and determined by the presence 

 or absence of barriers, which may be of many kinds. There may 

 be simple physical barriers like the sea, or a mountain chain; 

 there may be the barrier of a climatic change from warm to cold, 

 or from dry to wet, and so on; there may be ecological barriers 

 imposed by the habit or other peculiarities of the plant itself, and 

 so on. The whole quesliion is discussed in detail in chap, v (p. 32) 

 of Age and Area. 



So axiomatic did all this seem, that the author was somewhat 

 surprised by the vehement opposition that it encountered. The 

 explanation of this perhaps lies in the fact that geographical 

 distribution would thus be transferred to a more mechanical 

 sphere than had hitherto been allotted to it. No longer, especially 

 in view of the regular arithmetical arrangement, could the natural 

 selection theory supply a full explanation of the facts of evolution 

 into genera and species, and no longer, in face of the fact of 

 increase in number do^\Tiwards in the case of endemics, upwards 

 in case of wides (table on p. 29), could it supply a full 

 explanation of the facts of distribution, or of the nature of 

 endemics. Sooner or later, it seemed to the author, these new 

 discoveries meant that natural selection, in its present form at 

 any rate, would cease to be so important a factor in evolution, 

 and with evolution of course went distribution and many other 

 branches of biological science. 



One of the most important things that would necessarily 

 follow from the acceptance of age and area was the replacement 

 that it asked of the long-cherished notion that endemics in general 

 were either relic forms, or local adaptations, by the supposition 

 that when they occurred in very small areas they were mostly 

 young beginners as species, that had not yet had time to occupy 

 larger areas. In many cases of course barriers (especially barriers 

 due to climatic or soil conditions) that would in any event obstruct 

 or prevent further spread were so close that only small areas 

 could be covered, even though the species might be very old. 

 Other species, again, of very limited distribution, and that more 

 especially in the north within reach of the effects of the cold of 

 the last glacial periods, were evidently relics. Sinnott (Age and 

 Area, p. 86) gives, as examples of this class in North America, 

 Carya, Platiera, Madura, Garrya, Sassafras, Xanthorhiza, 



