260 THE SIGNIFICANCE OF SEXUAL REPRODUCTION 



by the ingenious investigations of J. Sachs (see Appendix III, 

 p. 308). We have not yet reached the limits of investigation, and no 

 reason can be assigned for the belief that we shall not some day 

 receive an explanation of characters which are now unintelligible 1 . 



It is obvious that the zoologist cannot lay too much stress upon 

 the intimate connexion between form and function, a connexion 

 which extends to the minutest details : it is almost impossible to 

 insist too much upon the perfect manner in which adaptation to 

 certain conditions of life is carried out in the animal body. In 

 the animal body we find nothing without a meaning, nothing 

 which might be otherwise ; each organ, even each cell or part of 

 a cell is, as it were, tuned for the special part it has to perform 

 in relation to the surroundings. 



It is true that we are as yet unable to explain the adaptive 

 character of every structure in any single species, but whenever 

 we succeed in making out the significance of a structure, it 

 always proves to be a fresh example of adaptation. Any one who 

 has attempted to study the structure of a species in detail, and to 

 account for the relation of its parts to the functions of the whole, 

 will be altogether inclined to believe with me that everything 

 depends upon adaptation. There is no part of the body of an 

 individual or of any of its ancestors, not even the minutest and 

 most insignificant part, which has arisen in any other way than 

 under the influence of the conditions of life ; and the parts of the 

 body conform to these conditions, as the channel of a river is 

 shaped by the stream which flows over it. 



These are indeed only convictions, not real proofs ; for we are not 

 yet sufficiently intimately acquainted with any species to be able 

 to recognize the nature and meaning of all the details of its struc- 

 ture, in all their relations : and we are still less able to trace the 

 ancestral history in each case, and to make out the origin of those 

 structures of which the presence in the descendants depends pri- 

 marily upon heredity. But already a fair advance towards the 

 attainment of inductive proof has been made ; for the number of 

 adaptations which have been established is now very large and 



1 Since the above was written many other morphological peculiarities of plants 

 have been rightly explained as adaptations. Compare, for instance, the invt sti^i- 

 tions of Stahl on the means by which plants protect themselves against the attacks 

 of snails and slugs (Jena, 1888). A. W., 1888. 



