ON THE MIXED HERBAGE OF PERMANENT MEADOW. 
1225 
proportion as the number of species is also small. The diminution not only in the 
number of species, but in that of individual plants, is usually the result of conditions 
favouring the luxuriance of some species more than others ; and hence the competition 
between plant and plant becomes more severe. The variety and the number are 
reduced, but the strength and size of those which remain are enhanced. The experi¬ 
mental details given in the sequel will afford some striking illustrations of these facts. 
Naegeli* considers the internecine war to be the most severe between those species 
and varieties which are the most nearly allied in character and organisation, because 
they require the same conditions of existence. He cites in illustration the case of 
three species of Achillea , two of which, A. moschata and A. atrata , make similar 
demands on their environment, and so come into competition one with another, with 
the result that the two are rarely found together, because one overpowers the other. 
The third species, A. Millefolium, having a different organisation from the other two is 
not brought into competition with them because its requirements are different; and it 
is, in consequence, found growing with either or both of the others. It must be 
remembered, however, that plants very closely allied morphologically, so far as their 
outward conformation is concerned, may, nevertheless, vary greatly in physiological 
endowments. We see this in the very varied constitutional and physiological peculi¬ 
arities sometimes manifested in seedlings derived from the same parent plant. We 
shall hereafter have to dwell at length on the marked physiological diversities between 
two nearly allied species of Poa , P. trivialis and P. pratensis, as also between three 
species of Avena already mentioned, but in these cases there are structural differences 
between the species which indicate a more distant relationship between them than is 
apparent at first sight. 
Naegeli further alludes in some detail to the manner in which the preponderance of 
one species and the consequent ultimate extinction of another nearly allied form may 
be brought about. “It might be assumed,” says Naegeli, “that this result would 
always take place, and that one of two plants would always be crowded out, because the 
two could hardly be precisely equally hardy. But this conclusion would be unsound, 
because it would hold good only for plants whose conditions of existence were as 
nearly as possible alike. We can imagine another case in which the two species suffer 
injury from altogether dissimilar external influences (one, e.g., from spring frost, the 
other from dry heat,) so that sometimes the number of individuals of one species, 
sometimes that of the other species diminishes, and where, moreover, the production 
and the germination of the seeds are affected by altogether dissimilar external in¬ 
fluences, so that sometimes the one, sometimes the other species, increases most 
rapidly and occupies the vacant spots. The numerical proportion of the two species 
must in this case be variable, but neither is able to expel the other.” The fluctua¬ 
tions from year to year of particular species on some of the plots at Rothamsted will 
afford similar illustrations. 
* Sitzungsb. der K, Bay. Acad, der Wissenscli., Dec. 15, 1865. See also Sachs’ ‘Text Book,’ 
English edition, 1875, p. 833. 
