SPONTANEOUS GENERATION AND EVOLUTION : CONCLUSION 385 



up the story, of which we have given a mere outline. I simply wish to 

 point out that in parasitic animals there is a vast range of forms of life 

 in which the most precise adaptation to the conditions occurs in almost 

 every organ, and certainly at every stage of life, in the most con- 

 spicuous and distinct manner. In the earlier part of these lectures we 

 gained from the study of the diverse protective means by which plants 

 and animals secure their existence the impression that whatever is 

 suited to its end {Das Ziveckmdssige) does not depend upon chance for 

 its origin, but that every adaptation which lies at all within the 

 possibilities of a species will arise if there is any occasion for it. 

 This impression is notably strengthened when we think of the life- 

 history of parasites, and we shall find that our view of adaptations as 

 arising, not through the selection of indefinite variations, but through 

 that of variations in a definite direction, will be confirmed. Adaptations 

 so diverse, and succeeding one another in such an unfailing order as 

 those in the life-history of a tape-worm, a liver-fluke, or a Sacculina, 

 cannot possibly depend upon pure chance. 



Nevertheless, chance does play a part in adaptations and species- 

 transformations, and that not only in relation to the fundamental 

 processes within the germ-plasm, but also in connexion with the higher 

 stages of the processes of selection, as I have already briefly indicated. 

 After the publication of my hypothesis of germinal selection it was 

 triumphantly pointed out that I had at last been obliged to admit 

 a phyletic evolutionary force, the ' definitely directed ' variation of 

 Nageli and Askenazy. This reproach — if to allow oneself to be con- 

 vinced be a reproach — is based upon a serious misunderstanding. 

 My ' variation in a definite direction ' does not refer to the evolution 

 of the organic world as a whole. I do not suppose, as Nageli did, 

 that this would have turned out essentially as it has actually done, 

 even although the conditions of life or their succession upon the earth 

 had been totally different ; I believe that the organic world, its classes 

 and orders, its families and species, would have diflfered from those 

 that have actually existed, both in succession and appearance, in pro- 

 portion as the conditions of life were different. My 'variation in 

 a definite direction ' is not predetermined from the beginning, is not, 

 so to speak, exclusive, but is many-sided; each determinant of a 

 germ-plasm may vary in a plus or minus direction, and may continue 

 under certain circumstances in the direction once begun, but its com- 

 ponents, the different biophors, may do the same, and so likewise may 

 the groups, larger and smaller, of biophors which form the primordia 

 (Anlagen) of the organs within the germ-plasm. Thus an enormously 

 large number of variational tendencies is available for every part of 

 IX. c c 



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