256 PHYSIOLOGICAL DEVELOPMENT. 



matter, is primarily due to the unlike actions of these unlike 

 parts of the environment, is, then, clearly implied by observed 

 facts more clearly, indeed, than was to be expected. Con- 

 sidering how strong must be the inherited tendency of a plant 

 to assume those special characters, physiological as well as 

 morphological, which have resulted from an enormous accu- 

 mulation of antecedent actions, it may be even thought 

 surprising that this tendency can be counteracted to so great 

 an extent by changed conditions. Such a degree of modifi- 

 ability becomes comprehensible only when we remember 

 how little a plant's functions are integrated, and how much, 

 therefore, the functions going on in each part may be altered 

 without having to overcome the momentum of the functions 

 throughout the whole plant. But this modifiability being as 

 great as it is, we can have no difficulty in understanding 

 how, by the cumulative aid of natural selection, this primary 

 differentiation of the surface in plants has become what we 

 see it. 



273. We will leave now these contrasts between the free 

 surfaces of plants and their attached or imbedded surfaces, 

 and turn our attention to the secondary contrasts existing 

 between different parts of their free surfaces. Were a full 

 statement of the evidence practicable, it would be proper 

 here to dwell on that which is furnished by the inferior 

 classes. It might be pointed out in detail that where, as 

 among the Algae, the free surfaces are not dissimilarly con- 

 ditioned, there is no systematic differentiation of them that 

 the frond of an Ulva, the ribbon-shaped divisions of a 

 Laminaria, and the dichotomous expansions of the Fuci 

 which clothe the rocks between tide-marks, are alike on both 

 sides; because, swayed about in all directions as they are by 

 the waves and tides, their sides are equally affected. Con- 

 versely, from the Fungi might be drawn abundant proof that 

 even among Thallophytes, unlikenesses arise between different 

 parts of the free surfaces when their circumstances are unlike. 



