VARIATION AND HEREDITY 35 



cell-sap may exercise a directive influence upon development, and thus 

 cause an individual variation to appear and be maintained. 



A detailed discussion of these and related questions would be beyond 

 the scope of this book. It must suffice to indicate the relationships which 

 are of importance in every case in inducing specific shape and character ; 

 relationships, moreover, to which sufficient attention is rarely paid in 

 attempts to explain the causes of the appearances observed. Whether 

 the reduction of the protoplast to its living essentials increases or di- 

 minishes the tendency to variation, and what may be the action of the 

 inductive and directive influences of the living environment, cannot be 

 discussed here. The development of the fertilized egg-cell within the 

 embryo sac is completed under peculiar specific conditions, and it is, perhaps, 

 owing to the influence which these exert, that the vegetative outgrowths 

 which sometimes arise from the wall of the embryo sac of Funkia, Coelobogytie, 

 &c., assume a similar shape and form to the sexually produced embryos. 



It would be beyond the scope of this book to do more than indicate 

 briefly a few of the general principles which regulate vital phenomena. 

 Phylogenetic considerations have been purposely neglected, nor has any 

 attention been paid to those theories which are based upon the existence 

 of some presupposed structure in protoplasts, and upon the assumption 

 of a definite division of labour corresponding to this structure. For 

 although we should strive to explain everything by direct reference to 

 the protoplasmic mechanism, our knowledge is as yet quite insufficient 

 to enable us to predict the actual results that will follow as the necessary 

 consequence of the given dispositions. This being the case, while allowing 

 complete freedom of thought to theoretical conceptions, it is essential in 

 every exact research to keep clearly in view the distinction between 

 established facts and theoretical possibilities. When our knowledge of the 

 nature of life is complete it will probably be found that the facts which 

 we know at present form the framework supporting the completed picture. 



In a subsequent consideration of the structure and mechanism of 

 the protoplast, the question of heredity will be reopened. So much at 

 least is clear, that every protoplast or part of a protoplast, which has 

 the power of reproducing the entire plant, must contain in itself all that 

 is necessary for the maintenance and development of the species. It does 

 not, however, follow that under actually existing conditions these potential 

 powers are, or can be, exercised. 



Starting with the physiological knowledge we already possess, and 

 especially by means of facts derived from the rapidly living lower organ- 

 isms, it will doubtless be possible to obtain a deeper insight into the causes 

 which originate variations and the manner in which these causes act. 

 With the increased knowledge then at our disposal, we may hope to throw 

 some light upon those variations which, according to the theory of descent, 



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