The ‘Age and Area’ Hypothesis and the Problem 
of Endemism. 
BY 
EDMUND W. SINNOTT, 
Connecticut Agricultural College. 
I N a series of recent papers Dr. Willis 2 > 3 has brought forward an 
hypothesis as to plant distribution which is called by him that of ‘ age 
and area ’ and which is based chiefly on his studies of the distribution and 
affinities of the floras of Ceylon and New Zealand. This hypothesis may be 
summarized briefly as follows : The area occupied by a given species in 
a given region in which there occur no well-marked barriers depends in the 
main upon the age of that species in that region — the older the species, the 
wider its range. This necessarily involves also the hypothesis that the ‘dying 
out * of a species happens very rarely indeed, and then, it is believed, only by 
‘ accident 5 or as the result of a geological convulsion or other important 
environmental change. 
The importance of this interesting hypothesis, in support of which 
Dr. Willis has brought forward an abundance of evidence, would obviously 
be very great if it should be proved correct. It would upset the traditional 
belief in Natural Selection as the most important factor in determining 
distribution. It would make it possible to tell at a glance the relative anti- 
quity of the various elements in any flora and thus to reconstruct with ease 
the phytogeographical history of a region. It would enable us to identify 
the most widespread species in a given genus or the most widespread genus 
in a given family as the most ancient type in that particular genus or family, 
and thus to clear up at once many vexatious problems of phylogeny. Perhaps 
no other single hypothesis bears directly upon such a multitude of problems, 
and its verification is consequently a matter well worthy of our attention. 
The purpose of the present paper is to bring forward certain facts which 
1 Willis, J. C. : The Endemic Flora of Ceylon, with reference to Geographical Distribution and 
Evolution in general. Phil. Trans., B, vol. ccvi, 1915, p. 307. 
2 Ibid. : The Evolution of Species in Ceylon, with reference to the Dying Out of Species. 
Ann. of Bot., vol. xxx, 1916, p. 1. 
8 Ibid. : The Distribution of Species in New Zealand. Ann. of Bot., xxx, 1916, p. 437. 
[Annals of Botany, Vol. XXXI. No. CXXII. April, 1917.] 
