310 EMBRYOLOGY 



as representatives of species, and nothing more, but all the living 

 beings that we know practically, we know as individual members of 

 their kind. We may, for our own purposes, and in our minds, consider 

 their kinship apart from their individuality, but this does not show 

 that they are separable in fact. Living beings do not exhibit unity 

 and diversity, but unity in diversity. These are not two facts, but one, 

 and the separation is in our minds, or our words, and not in nature. 

 The delight of intimate acquaintance with animals is due to the in- 

 separableness of their specific unity from their individuality, and our 

 attempts to separate, in our minds, what is not separable in fact, 

 lead us to two narrow and partial views of the facts, two crude and 

 imperfect mental concepts, neither of which corresponds to anything 

 in nature. 



All this is familiar, but I ask you to reflect upon it; to decide for 

 yourselves whether it does not mean that inheritance, or resemblance 

 to ancestors, and variation, or difference from ancestors, are only 

 imperfect mental concepts, crude ideas, and not facts; whether the 

 fact is not the individuality in kinship of living beings. Each of you 

 must answer this simple question for himself. I cannot regard them 

 as facts, since they seem to me to be only imperfect ideas of facts; 

 mental states which we have reached by fixing our attention upon 

 a partial and uncritical view of our sensations and perceptions, to the 

 neglect of that which has not interested us nor seemed to concern us. 



If you agree with me that resemblance to ancestors does not exist 

 in nature apart from individuality or difference from ancestors, that 

 inheritance is not a fact, but only an imperfect idea of a fact, admitting 

 of correction and improvement by comparison with nature, and in 

 no other way, if you agree to this, what becomes of the notion of 

 a substance of inheritance? There is, no doubt, a material equivalent 

 for every idea, but the material equivalent for the notion of a sub- 

 stance of inheritance is in the brain of the speculative philosopher, 

 and not in germ-cells. St. Paul says faith is the substance of things 

 hoped for, the evidence for things not seen, but in science the evi- 

 dence for things not seen is to be sought by using our eyes, and I can 

 discover no basis for the notion of a substance of inheritance except 

 faith, because inheritance seems to me to be a term to designate a 

 narrow and imperfect view of facts, and not itself a fact. 



I hope you will not accuse me of opposing the scientific study of 

 inheritance and variation, for nothing is farther from my thoughts. 

 The resemblances and differences between ancestors and descendants 

 are as worthy of study as arithmetic, which has been of inestimable 

 value to mankind, although there is, in nature, no quantity without 

 quality. We cannot make any progress in natural knowledge without 

 fixing our attention upon some narrow and imperfect view of nature, 

 to the temporary neglect of that which does not interest us, but things 



