158 INTRODUCTION TO GENERAL PHYSIOLOGY 



it is well known that there are differences of a more or less marked 

 degree. These " variations " are often called " spontaneous," not as 

 implying that they are self-produced by a so-called inherent 

 tendency to vary, but that we cannot, as yet, assign an actual cause 

 to them. In any case, they are made use of by natural selection 

 for the production of new races and species. Darwin speaks of 

 " The Origin of Species by means of natural selection, or the 

 preservation of favoured races in the struggle for life," and he sums 

 up the general theory as follows : " As many more individuals of 

 each species are born than can possibly survive, and as, conse- 

 quently, there is frequently recurring struggle for existence, it 

 follows that any being, if it vary however slightly in any manner 

 profitable to itself, under the complex and sometimes varying con- 

 ditions of life, will have a better chance of surviving, and thus be 

 naturally selected. From the strong principle of inheritance, any 

 selected variety will tend to propagate its new and modified form." 

 The last sentence may be also put thus : An individual which shows 

 a new character is more likely in general to leave progeny possess- 

 ing this character than are those without it. If it is one that 

 enables its possessor to make better use of the forces of the 

 environment, this individual is better situated, especially by its 

 longer life, to leave more progeny, thus increasing the probability 

 of the permanence of the new character. 



Adaptation. We see that, according to the view expressed in 

 the last paragraph, the " adaptation " of organisms to their sur- 

 roundings, or their fitness to their environment is indirect. On the 

 other hand, certain writers have strongly advocated the existence 

 of direct adaptation. No really conclusive proof of any case has 

 yet been brought forward, and if we consider what it means, it 

 must be admitted that it seems very unlikely. It would mean that 

 the reaction of an organism to a new influence is such that the 

 effect of this influence is to produce either a means of making 

 appropriate use of it, or of meeting it, if injurious, by an appro- 

 priate defence. Since the reactions of an organism are necessarily 

 conditioned by its structure and properties, it is difficult to see 

 how a totally new condition would find the capacity of an appro- 

 priate reaction. The reaction may chance to be a favourable one, 

 but that is not direct adaptation. The only strong evidence is 

 derived from two characteristics of bacteria. The first is, that 

 in certain cases bacteria grown on a medium, which they 

 are unable to utilise as food, gradually develop in successive 

 generations the power to clo so. The weak point here is that we 

 are dealing with several generations, and we cannot be sure that in 

 the first culture there might not be a few individuals capable of 

 utilising to a small extent the new food material. If so, these would 



