INTRODUCTION TO THE METHOD 



for the origin of species in the singular, but a painstaking analysis of numerous special 

 cases o f the ori.g ins of species in the plural, carried out objecli\cly and without undue 

 deductive reasoning until sufficient wealth of well-authenticated individual cases should 

 have been assembled to make a fresh generalization possible. This programme of 

 work took the rest of the twentieth century to carry out and was still occupying the 

 ^attention of many minds in the middle of the twenty-first. 



'By this time', our historian might continue, 'two further events had occurred. The 

 effect of the visual light micros cope on the scope of work of the nineteenth century was 

 repeated at a different level vijijthe twentieth by the perfecting of the electron micro- 

 scope. This brought the field of molecular structure under direct observation, and 

 made possible for the first time a full description of organic materials. Simultaneously, 

 a synthesis was effected between cytogenetics and the previously independent science 

 of enzymology, so that the dynamics of living matter could at last be explored. The 

 consequences of these developments were naturally jiot fully appreciated until the 

 _twenty-first century was well under way, although the shadow of big things to come was 

 clearly discernible by the middle of the twentieth.' 



Taking leave of our historian while yet there is time and before he intoxicates us 

 with a preview of distant scenes, we may ask in sober earnest what has in fact happened 

 to the theoretical background of knowledge in almost a century which has elapsed 

 between publication of The Origin of Species in 1859 and the present day? As I see 

 it, the change in our attitude to theories of evolution rests principally in a new precision 

 which can now be attached to the words 'variation' and 'variability'. In Darwin's 

 day these concepts were so ill-defined that plausible assumptions, uncontrolled by any 

 reference to experiment, could at any time be made without serious challenge except 

 on a priori grounds. At present we possess enough exact knowledge about both terms 

 to be able to discuss the basic concepts of this or that general theory of evolution, not on 

 logical grounds alone, but to some extent on a basis of fact. The underlying assump- 

 tions required by simple Lamarckism (evolution by use and disuse), or simple Darwin- 

 ism (adaptive evolution by gradual accumulation of minute heritable differences) 

 or the mutation theory first voiced by Darwin's earliest opponent, Richard Owen, and 

 later much elaborated in the hands of de Vries and others (evolution by sudden leaps), 

 can all now be seen in their original form to be over-simplifications. They may con- 

 tain a greater or less germ of truth, but they cannot be the whole truth, for we now 

 know enough to be certain that variation is not one process but many; different types 

 of variation have widely different causes and consequences, and all follow their own 

 laws of behaviour which must first be elucidated before they can safely be built into any 

 theoretical scheme. The task before us is therefore seen to be something quite different 

 from that with which the theorists of the last century were concerned. We have a 

 new field of knowledge to explore, and the exploration is only just beginning. Exactly 

 where it will end cannot at this date be wholly foreseen, and it may therefore be well 

 at the outset to disinterest ourselves from general theories in order to concentrate the 

 better on a limited number of rather fundamental questions. How many types of 

 evolutionary activity can we actually detect? How do these differ and what are their 

 characteristics? What proportion do the analysable cases bear to the unanalysable? 



Q i-a 



