24 INTRODUCTION. 



Wiih the differences of dififerentiation of the meristem are always connected 

 those of the mature structure : one may say obviously so, since the causes of the 

 development of the mature structure, which are involved in the properties of the 

 meristem, are different in every case. 



But while the differences in the differentiation of the meristem correspond in 

 each case exactly to systematic divisions, as d'stinguished principally on the ground 

 of other phenomena, and especially so in the greater groups — for instance, all Ferns 

 and Equisetums correspond just as closely, and are distinguished from other classes 

 just as much by the differentiation of the apex of their stem and root, as by their 

 reproductive and embryonic processes — the case is often different with the mature 

 structure. The structure of the full-grown stem of Equisetum has no more 

 resemblance to that of a Fern than to that of any Angiospermous plant however 

 distantly related, as regards both external members and internal structure. Similar 

 divergences are found on all sides between the characters of mature plants, and 

 the embryonic or meristematic stages which indicate their relationships. Conversely, 

 there is equally often to be found a convergence of properties of distantly connected 

 species : and this is clearly expressed externally in the similarity of the most 

 heterogeneous plants which live under like conditions, such as water-plants, the 

 vegetation of steppes, and shores, &c. 



The reason of these phenomena is easily understood when seen from the point 

 of view of the theory of Descent, and has often enough been stated. The existing 

 form of a species is determined by the inheritance of the properties of its ancestors, 

 a;nd by the changes which these properties undergo through the influence of the 

 environment, i.e. the adaptation to the latter. The inherited properties must remain 

 most clearly retained in those stages of ontogenetic development which through all 

 generations are most independent of, that is most protected from external influences, 

 and this is the case with the embryonic and meristematic stages. These recall in 

 each species most completely and clearly the whole series of its reminiscences of 

 descent, or, what is the same, they are more plainly different according to the 

 divisions of the natural system than the later stages. Certainly these are also 

 influenced by heredity, but the results of this may be obliterated and diverted from 

 the original direction by successively accumulated adaptation. 



As in the external conformation, which must at most be only incidentally touched 

 upon here, so in the internal structure and arrangement of the tissues we may accord- 

 ingly distinguish two series of phenomena. On the one hand those in which we recog- 

 nise the direct effects of the environment {phenomena of direct adaptatioti), since they 

 appear in plants of the most different affinity, as soon as they are adapted to like con- 

 ditions of life ; and since they may change with these conditions of life even in the 

 same individual. It is hardly necessary to cite as proof the different forms of growth, 

 which recur in the most unlike circles of affinity, and the anatomical peculiarities con- 

 nected with these ; or the remarkable similarity between species of the same habitat, 

 which are systematically as far apart as possible : of the latter we may quote as the most 

 prominent examples, in the first place, the water-plants, the similarities of which are 

 independent of their systematic position, and will be often referred to in the follow- 

 ing chapters. In the amphibious species the most remarkable varieties of general 

 conformation appear, according as an individual, or even a part of one, lives in or out 



