2.22 THE EVOLUTION THEOKY 



case there must be growth subsequent to the conjugation lief ore the 

 normal is re-attained. It may 1)e doubted, therefore, whether the 

 increase in mass is, even in the case cited, the essential event in 

 conjugation, and whether there are not other effects which we cannot 

 clearly recognize. Here, too, there must be difierences between the 

 two conjugating individuals, as we ha\'e just seen, for if they only 

 communicated something similar to each other, the result would be an 

 increase only in tlieir ma.ss, not in their cjualities. 



Althoup-h we cannot demonstrate differences of tliis kind in the 

 case of the lowly organisms with which we are now dealing, we may 

 assume their existence from analogy witli the higher organisms. 

 We know, especially through O. Jager, that in Man every individual 

 lias a specific exhalation, his particular odour, and that in the secretions 

 of his glands there are incalculably minute difierences in chemical 

 composition, which justify the conclusion that the living substance 

 of the secreting cells themselves exhibits such differences, and that 

 all the various kinds of cells in an individual are not absolutely 

 identical with the corresponding cells of another individual, but that 

 thej^ are distinguished from them by minute yet constant chemical 

 differences. The assumption that differences of this kind exist even 

 in unicellulars, and in all lowly organisms generally, is not a merely 

 fanciful one, but has much probability. 



How far the combination of these individual differences of 

 chemical, and at the same time vital, organization is able to quicken, 

 to streng-then the metabolism, to bring about 'physiological regene- 

 ration,' or whatever we may choose to call it, we do not yet understand. 

 It has been said that in plastogamy an exchange of ' substances ' takes 

 place ; that each gives to the other the substances which it possesses 

 and the other lacks, and that this causes an increase of vital energy. 

 But it is unlikely that we have liere to do merely with chemical 

 substances, although these, of course, as the material basis of all vital 

 processes, are indispensable ; it seems to me more probable that the 

 vital units (biophors) themselves in their specific individuality must 

 play the chief part. But even this is saying very little, for we have 

 not yet reached an understanding of these processes, and if we were 

 not forced by the fact of plastogamy to the conclusion that this union 

 must have some use, no one would have been likely to postulate 

 it as useful, still less as necessary. It has, of course, been frequently 

 suggested that multiplication by fission, if long-continued, results 

 in 'exhaustion,' and that this is corrected by amphimixis, but who 

 can tell why this ' exhaustion ' might not be remedied, and even more 

 effectually remedied, by a fresh supply of fuel, tliat is, of food ? One 



