458 



SCIENCE 



[N. S. Vol. XLVI. No. 1193 



Barriers of various sorts certainly do exist 

 almost everywhere and effectively limit the 

 extent to which a species may be dispersed. 

 We have reason to believe that many types 

 are as widespread as they can ever be and that 

 no increase in age, other factors remaining 

 constant, will widen their ranges. In fact, 

 evidence from fossils shows that certain spe- 

 cies and genera occupy to-day smaller areas 

 than they formerly did. 



Factors inherent in the plant itself are also 

 bound to influence the extent of its distribu- 

 tion. Types which are hardy and able to 

 thrive under a wide range of conditions will 

 obviously spread farther and faster that those 

 which are sensitive or specialized. The growth 

 habit of a plant, too, seems to be very im- 

 portant in determining distribution, trees usu- 

 ally occupying small ranges, shrubs wider 

 ones and herbs the widest of all. This may 

 be observed in almost any flora and is very 

 noticeable in those of Ceylon and ISTew Zea- 

 land, where the endemic species, necessarily 

 of limited dispersal, are predominantly trees 

 and shrubs ; the non-endemic, widespread ones, 

 predominantly herbs. The data as to rela- 

 tive commonness of species in Ceylon given 

 in Trimen's "Flora," the authority used by 

 Professor Willis, also show clearly that the 

 herbs are much commoner and more widely 

 dispersed than are the woody plants. 



The theory that the most widespread types 

 are the oldest meets with further difficulties 

 from some of its implications. The fact 

 which we have just mentioned, that species 

 of herbs tend universally to have much wider 

 ranges than those of shrubs or trees, a cir- 

 cumstance long ago noted and emphasized by 

 De Candolle, must mean, if we follow Pro- 

 fessor Willis, that the herbaceous element in 

 the angiospermous vegetation of the globe is 

 more ancient than the woody element. 

 Against this conclusion there are serious ob- 

 jections, and it is at present maintained by 

 few botanists or geologists. In its interpre- 

 tation of endemic types the hypothesis is also 

 open to objection, since it regards endemic 

 species and genera in all cases as of recent 

 origin, the newest element in their respective 



floras. There is much evidence, however, 

 from taxonomy and paleobotany, that in many 

 cases endemics are relicts of types once much 

 more widely spread which have disappeared 

 from all regions save one. Such endemics 

 are evidently ancient rather than recently 

 acquired members of a flora. 



This point involves the necessary corollary 

 to his hypothesis which Professor Willis 

 brings forward when he states^ that the 

 " dying out " of a species is a rather rare event, 

 usually requiring some profound geological or 

 climatic change. This belief in the essential 

 permanency of types necessarily leads Pro- 

 fessor Willis to the view that species or genera 

 which are isolated taxonomically and without 

 near relatives have become so not through the 

 extinction of intermediate and connecting 

 forms, but by a single step, a view demanding 

 belief in the frequency and permanence of wide 

 mutations. If we look again at the fossil 

 record, however, we see such an overwhelming 

 array of extinct types that it is hard to attrib- 

 ute their extermination in every case to a 

 cataclysmic disturbance. This difficulty in- 

 creases when we examine the flora of any such 

 isolated region as Ceylon or New Zealand. If 

 Professor Willis's hypothesis is correct, the 

 original invaders of ea«h of these islands — its 

 oldest plant inhabitants — should now be the 

 most widespread and common members of its 

 flora, in contrast to the endemic forms which 

 have sprung from them and are thus more rare 

 and local. If we look at the flora of Ceylon, 

 however, we find that there are no less than 63 

 genera of dicotyledons alone, 8 per cent, of the 

 whole, which, though not endemic in Ceylon, 

 are represented only by endemic species. In 

 New Zealand 90 non-endemic genera of dicoty- 

 ledons, or 43 per cent, of the whole, are simi- 

 larly represented only by endemic species. In 

 these cases, where in each genus is the parent 

 species or group of species, the original in- 

 vader, which has supposedly given rise to all 

 these endemic forms and which should now be 

 more common than any of them? It certainly 



2 WiUis, J. C, ' ' The evolution of species in 

 Ceylon, with reference to the dying out of spe- 

 cies," Annals of Botany, Vol. XXX., 1916, p. 1. 



