RELATION OF MORPHOLOGICAL NATURE OF ORGANS TO ADAPTATION. 933 
^. moschata and 330,000'^. atrata. After a second frost, which again destroys one half 
of the individuals of A. atrata, we should have about 800,000 of A. moschata to 200,000 
of A. atrata. In this manner the number of the latter would decrease with every 
unusual summer frost, until at length it entirely disappeared, a nearly-allied hardier species 
becoming distributed over the locality in its place.' In conclusion, the following remark 
by the same author may be added : — ' From such a course of reasoning the conclusion 
might perhaps be drawn 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 influences, so that sometimes the one sometimes the 
other species increases more 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.' 
Just as the struggle between two species is the result of their thriving more or less 
vigorously on a soil of a particular chemical nature, so also the need for more or less 
water, light, heat, &c. can determine also the nature of the struggle for existence. 
Nägeli gives some examples of the first case. When Primula officinalis and elatior occur 
together in a district, they are sometimes sharply separated from one another, P. offici- 
nalis preferring the dry, P. elatior the damp spots. Each is most vigorous in its own 
locality, and may expel the other. But when only one species occurs, it is not so 
particular ; P. officinalis will choose damper, P. elatior drier situations, than if they were 
in company. Prunella 'vulgaris and grandiflora behave in the same manner in reference 
to poorer and more fertile soils ; as also do Rhinanthus Alectorolophus and minor, Hieracium 
Pilosella and hoppeanum. 
These examples may suffice to show what is meant by the Struggle for Existence. It 
must however be borne in mind that such a struggle must arise in reference to every 
vital phenomenon of a plant, and to each of its relationships to the external world, 
especially to the animal kingdom; and that its course must vary for the same plant 
in different localities. An understanding of the Theory of Descent, and especially 
an insight into the causes of the perfect adaptations of the organisation of the plant 
to its vital conditions which are often extremely local, depend essentially on a clear 
comprehension of the struggle for existence. 
Sect. 38. — Relationship of the morphological nature of the organ to 
its adaptation to the conditions of plant-life. Every plant is very accurately 
adapted (though not absolutely so) to the conditions and circumstances under which 
it grows and is reproduced; its organs have the shape, size, mode of develop- 
ment, power of movement, chemical properties, &c. needful for this purpose. If 
this were not the case, the plant would inevitably perish in the struggle for existence. 
But the vital conditions are extremely various, and undergo, in the course of time, 
endless changes. The diversity in the characters of plants corresponds to this infinite 
variety in the conditions of Hfe ; and yet even in the more highly differentiated classes 
there are only three or four morphologically distinct forms of structure, axis (cau- 
lomes), leaves (phyllomes), roots, and trichomes, which suffice for these conditions, 
while maintaining a constant morphological character through numberless variations 
in their physiological properties. This relationship has already been described in 
chap, iii of Book I as the metamorphosis of the morphological members of a plant, 
