LECTURE I 



INTRODUCTORY ?' 



Every one knows in a general way what is meant by the doctrine 

 of descent — that it is the theory which maintains that the forms of 

 life, animals and plants, which we see on our earth to-day, have not 

 been the same from all time, but have been developed, by a process of 

 transformation, from others of an earlier age, and are in fact descended 

 from ancestors specifically different. According to this doctrine of 

 descent, the whole diversity of animals and plants owes its origin to 

 a transformation process, in the course of which the earliest in- 

 habitants of our earth, extremely simple forms of life, were in part 

 evolved in the course of time into forms of continually increasing 

 complexity of structure and efficiency of function, somewhat in the 

 same way as we can see every day, when any higher animal is 

 developed from a single cell, the egg-cell, not suddenly or directly, 

 but connected with its origin by a long series of ever more complex 

 transformation stages, each of which is the preparation for, and leads on 

 to the succeeding one. The theory of descent is thus a theory of 

 development or evolution. It does not merely, as earlier science did, 

 take for granted and describe existing forms of life, but regards them 

 as having become what they are through a process of evolution, and 

 it seeks to investigate the stages of this process, and to discover the 

 impelling forces that lie behind it. Briefly, the theory of descent is 

 an attempt at a scientific interpretation of the origin and diversity of 

 the animate world. 



In these lectures, therefore, we have not merely to show on what 

 grounds we make this postulate of an evolution process, and to 

 marshall the facts which necessitate it ; we must also try to penetrate 

 as far as possible towards the causes which bring about such trans- 

 formations. For this reason we are forced to go beyond the limits 

 of the theory of descent in the narrow sense, and to deal with 

 the general processes of life itself, especially with reproduction and 

 the closely associated problem of heredity. The transformation of 

 species can only be interpreted in one of two ways ; either it depends 

 on a peculiar internal force, which is usually only latent in the 

 organism, but from time to time becomes active, and then, to a 



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