168 The Philippine Journal of Science 10:6 
If Doctor Willis could see Stenochlaena areolaris where it 
thrives, he would surely be convinced that at least this plant 
is specifically adapted to its peculiar habitat. It is epiphytic 
on one species of Pandanus. Its adaptation to the very peculiar 
conditions presented by its “substratum” is such that it can 
grow nowhere else. Geographically, it is restricted, therefore, 
to the few square miles where Pandanus utilissimus occurs. 
I expect to describe this most extreme case of adaption at 
greater length. For the point under discussion here, equally 
valid evidence is presented by thousands of known species of 
fungi. Almost every species of parasitic fungus has one host 
species or a single group of host species, which it is able to 
attack. Is it imaginable (not to ask for a demonstration) that 
it is anything except specific adaptation of parasite to host — that 
is, specific adaptation of the fungus plant to its own peculiar 
environment — that lets the fungus attack its host, but not the 
infinitely more numerous other plants growing in the neighbor- 
hood? Such a question seems to answer itself. 
Finally, the last of Doctor Willis’s papers, so far published, 
deals with the dying out of species, and seeks to show that the 
extermination that must occur, if natural selection operates in 
the usually supposed manner, does not occur or seems not to 
be occurring among the plants of Ceylon. The body of the 
paper is chiefly a restatement of the facts in the more extensive 
paper in the Philosophical Transactions, the minor attention, 
given to the question of dying out, earning one paragraph out 
of eight in the summary. It may be, as Doctor Willis maintains, 
that his figures do not furnish any reason to suspect species of 
being on the downward grade. If there are no species dying 
out in Ceylon, the number of species in the island must be under- 
going a constant increase, and, indeed, this is probably happen- 
ing. Increase in the number of species must result in a de- 
creased average commonness — that is, abundance in indi- 
viduals — of all species. Otherwise, the number of individuals in 
Ceylon is increasing and this is not so probable. If no species 
is driven to the wall, while the average number of individuals 
of all species decreases, it is rather strange ; but it must certainly 
be expected that, as many new species are introduced or evolved, 
and some of these become very common, the disappearance of old 
species will be comparatively slow. In a study of the flora of 
four towns in southern Wisconsin, 10 in which particular atten- 
10 Shriner and Copeland. Deforestation and creek flow about Monroe, 
Wisconsin, Bot. Gaz. 37 (1904) 139-143. 
