32 INTRODUCTION 



same or in different directions. Such variations may become permanent 

 and hereditary, either a change of form or an alteration in the products 

 of metabolism taking place l . 



The production and hereditary transmission of such variations are 

 connected in many ways with the general physiological problems with 

 which we are immediately concerned. When any variation takes place, an 

 alteration in the structure or nature of the protoplast must have previously 

 occurred, provided the variation is not merely a temporarily induced 

 one, but is one capable of hereditary transmission to the offspring. This 

 is true for the lowest as well as the highest plants, and whether the 

 variation is perpetuated by sexual or asexual reproduction. The con- 

 clusion that a change of this kind necessarily indicates an alteration in 

 the arrangement or character of the protoplasmic constellation is, indeed, 

 a logical necessity, even though it is impossible to determine exactly how 

 the given variation arises or is induced. 



The production of hybrid forms is evidently due to the combination 

 of two different kinds of living substance. There can be no doubt that, 

 if it were possible to interchange the nuclei of two separate and distinct 

 protoplasts, assuming that the strange nuclei and protoplasts could live 

 and grow together, two new organisms would be produced, differing from 

 one another and from the original protoplasts. These special charac- 

 teristics of the new organisms would be preserved, so long as the union 

 and co-operation between the parts of the new protoplasts were maintained. 

 This would also be the case if, for example, a bacterium existed in 

 intimate and permanent symbiotic union with the protoplast, as a chloro- 

 plastid does, and were transmitted from generation to generation in the 

 ovules. It is, as a matter of fact, not inconceivable that the existence 

 of certain species as such depends upon protoplastic or symbiotic unions 

 of similar character to the above. Nor is the possibility excluded that 

 the tiny symbiont might be too small to be visible, or might be unable 

 to continue an independent existence outside of the protoplast. Com- 

 paratively recently, lichens were regarded as distinct organisms, although 

 we now know that they are the products of a synthetic union of two 

 distinct plants, and that, by the artificial synthesis of various algae and 

 fungi, new forms, or forms similar to those already known, may be pro- 

 duced with relative ease. 



Nevertheless, as is well known, variations capable of hereditary trans- 

 mission may arise without the help of foreign protoplasm, and certain bacteria 

 afford especially instructive examples of these. Thus in many bacteria, the 

 power of forming either spores or certain metabolic products may be in- 

 hibited by a particular mode of treatment, and in some cases this inhibition 



1 The literature upon these questions is too voluminous to be quoted. 



