June 28, 1918] 



SCIENCE 



629 



THE RELATIVE AGE OF ENDEMIC 

 SPECIES 



Is my previous reviews of the studies of J. 

 C. Willis on endemic species I have explained 

 his thesis that they are, as a rule, the youngest 

 components of a flora and that their special 

 condition is due to the circumstance that they 

 have not yet had time to spread. This conclu- 

 sion was based chiefly on statistical studies of 

 the angiospermous floras of Ceylon and New 

 Zealand. 



It was to be expected that this contention 

 wovild not escape contradiction, since among 

 the older botanists the opinion prevails that 

 endemic species are everywhere the relics of an 

 old flora which is now rapidly disappearing. 

 There is no doubt that in some countries this 

 may be the case, and that even in the islands 

 studied by WiUis some few species are in this 

 condition. But the number of these relics is 

 so small that whenever a flora is studied sta- 

 tistically, they have no visible effect on the 

 figures, provided that the species are dealt with 

 in groups of about twenty or more. 



The arguments against the hypothesis of 

 the relative youth of endemics have been col- 

 lected and brought forward in a recent article 

 by H. N". Ridley and replied to by Willis.' In 

 the first place it is claimed that it is difficult 

 to show from which other species of the same 

 flora the endemics should have been evolved. 

 But Willis answers that most of them have 

 " wides " of the same genus in their neighbor- 

 hood, the " wides " being those species which 

 also occur in adjoining countries, and are 

 usually widely spread. A study of the diag- 

 nostical differences of the endemics with these 

 goes to prove that in most cases they can 

 easily be derived from them. Moreover it 

 shows that these characters are by no means 

 of an adaptive nature since they do not betray 



1 H. N. Ridley, ' ' Endemism and the Mutation 

 Theory," Ann. Bot., Vol. XXX., 1916, p. 551, and 

 J. C. Willis, "The Relative Age of Endemic Spe- 

 cies and Other Controversial Points," in the same 

 journal. Vol. XXXI., 1917, p. 189. See also some 

 other articles of the same authors in that journal, 

 and my reviews in SciEKCE, N. S., Vol. 43, No. 

 1118, pp. 785-787, June, 1916, and Vol. 45, No. 

 1173, pp. 641-642, June, 1917. 



any relation to the life conditions of tlie local 

 environment. 



Ridley is an adherent of the Darwinian 

 principle that organisms produce varieties, 

 which if more suitable to the surrounding con- 

 dition than the parent form are selected. 

 This thesis, of course, is the basis of all evolu- 

 tionary theories. But the question whether 

 the production goes by infinitesimal steps or by 

 larger changes, makes the difference between 

 the theory of natural selection and that of mu- 

 tation. On this point his criticisms clearly 

 show that the more narrowly one looks into 

 the actual facts the larger becomes the evidence 

 against the older view. The reader will find 

 a valuable review of the arguments in the 

 papers of both antagonists, but it would take 

 me too far to consider them here. 



The main point of Willis's position, how- 

 ever, has been left unattacked. It is the sta- 

 tistical result that the endemics and the widely 

 distributed species in a country are arranged 

 in graduated series, showing an increase in 

 number in opposite directions, the endemics in- 

 creasing from those of wide to those of nar- 

 rower distribution, the wides in the other di- 

 rection. Or, in other words, the endemics of a 

 flora are the more numerous the smaller their 

 area is, whereas among the species occurring 

 also outside the special flora studied, those 

 with a wide disitribution within it, prevail. 

 The regularity with which these facts appear 

 from the tables made for different islands and 

 different botanical groups can not be explained 

 on the old view. Neither is this possible for 

 the fact that endemics have mostly contiguous 

 areas, whereas the dying out of species should 

 lead us to expect the occurrence of their last 

 relics on sundry spots and on distant points of 

 their original habitat. 



This law of distribution, which Willis calls 

 his law of " age and area " is then tested for 

 some other floras, besides those already men- 

 tioned.- The orchids of Jamaica give a very 

 convincing instance. Dividing the island into 

 19 equal squares and comparing the endemic 

 orchids with those found also in Cuba, and 



- J. G. Willis, ' ' Further Evidence for Age and 

 Area, its Applicability to the Ferns, etc.," Ann. 

 Bot., Vol. XXXI., 1917, p. 335. 



