i] INTRODUCTION 3 



and method of evolution arise therefore partly from 

 our ignorance of the laws of variation and heredity, 

 and partly from different ideas as to the causes which 

 lead to progression in certain directions rather than 

 in others. This latter source of disagreement is to a 

 large extent outside the province of this book, but 

 the subjects of Heredity and Variation are so inti- 

 mately bound together that one cannot be adequately 

 treated without the other. If, however, we can come 

 to any definite decision with regard to the nature of 

 Heredity and Variation, we shall have made a long 

 step towards understanding the method by which 

 evolution has takeu and is taking place. 



One other point must be mentioned. The study 

 of heredity brings us face to face with perhaps the 

 most fundamental problem of biology the ultimate 

 nature of living matter. For if an ovum, barely 

 visible to the eye, or the much smaller spermatozoon 

 which is visible only with high magnification, can 

 bear potentially all the parental characters which 

 may be inherited by the offspring, it is clear that 

 any hypothesis of the nature of living matter must 

 take these things into account ; and though we 

 cannot unravel or even imagine it, we can at least 

 get some idea of the amazing complexity of the 

 substances which in thoughtless moments we group 

 together under the single name of 'protoplasm/ 



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