CAUSES OF THE PROGRESSIVE DEVELOPMENT OF VARIETIES. S^^^ 



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 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.' 



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. 

 NageU 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 par- 

 ticular ; P. officinalis will choose damper, P. elatior drier situations, than if they were in 

 company. Prunella 'vulgaris and grandijiora 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 suflftce 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 

 na insight into the causes of the perfect structural contrivances adapted to the vital 

 conditions of the plant which are often extremely local, depends essentially on a 

 clear comprehension of the struggle for existence. 



Sect. 36. — 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 in 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 life ; 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, 

 understanding by metamorphosis the adaptation to various physiological purposes of 

 morphologically equivalent members. The diversity in the physiological development 

 is directed to the vital conditions of the plant ; and to this extent Metamorphosis is 

 synonymous with what we have here termed Adaptation, and which has also been 

 described as Accommodation. When we speak of Purpose in the structure of a 

 plant, we mean in fact nothing more than that the form or other characters of the 

 organ are adapted to its conditions of life, which may be at once inferred from the 



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