VARIATION AND HEREDITY 31 



or combinations, generally of extremely complex character. In this respect 

 irritability and irritable manifestations are of especial importance. 



Usually, either over-accumulation or deficiency, or, in more general 

 terms, any disturbance of equilibrium, acts as a stimulus, while in the 

 regulation of metabolic action the stimulatory influences exerted by mass, 

 quantity, and quality play a most important part. Very often regulatory 

 influences are exerted by enzymes or other chemical substances ; and, again, 

 just as the male element not only acts as a stimulus to developmental 

 processes in the ovum, but also influences their formative character, so also 

 is it possible that occasionally, or even perhaps very frequently, a special 

 formative activity may be induced by an accession of living plasmatic 

 particles derived from some other part of the organism. 



By means of the plasmatic threads which provide for interprotoplas'mic 

 communication from cell to cell, a continuity of the living substance is 

 maintained which is undoubtedly of the highest importance in ensuring the 

 harmonious co-operation of the whole. To what extent other factors, such 

 as a transference of material particles, are of importance in ensuring this 

 harmonious co-operation it is at present impossible to say. When one 

 considers, however, the manner in which the vibrations of a string may 

 be transferred to others at a distance, or how, by means of the telephone, 

 messages and commands may be caused to re-echo far away, it is difficult 

 to see any reason why special stimulating effects may not be transmitted 

 through the nerve-like plasmatic threads by means of vibratory impulses 

 to distant parts. It is, indeed, conceivable that impulses of all kinds, 

 simple and complex, may be transmitted in this manner, although it must 

 at present remain uncertain whether mechanical vibrations, heat-waves, 

 electric currents, &c., may or may not act as transmitting agents. 



SECTION 5. Variation and Heredity. 



In correspondence with the line of treatment we are pursuing, we must 

 restrict ourselves to the discussion of the functions and reactions of existing 

 organisms. The phylogeny of existing plants is also a physiological problem, 

 but one about which only the most fragmentary and disconnected evidence 

 has been collected. For this reason, the fundamental principles which 

 enable us to comprehend the past history and the evolutionary development 

 of existing species must be sought in a knowledge of the vital phenomena 

 which take place under our own eyes in contemporary plant-life. The 

 organisms of the present day are not immutable or unchangeable in character; 

 for, in addition to those alterations which take place during developmental 

 progress, accidental variations may appear, and may be repeated in the off- 

 spring, so that, under precisely similar external conditions, the descendants 

 may diverge more or less markedly from the common ancestral type in the 



