450 SCIENTIFIC THOUGHT. 



the three great generalisations mentioned above. Out 

 of the three ideas of pre-formation, after-formation, and 

 the directive principle, the three generalisations, namely, 

 the cellular theory, natural selection, and metabolism, 

 and the enormous number of facts collected by micro- 

 scopists and naturalists of all kinds, many more or 

 less ingenious theories of life have been put together. 

 None of them has obtained, though some have had a 

 very marked influence on biological science, and even 

 48. on popular thought. Of these Prof. Weismann's theories 



Weismann 



on heredity. o f heredity are probably the best known. Without en- 

 tering upon the enormous array of biological facts which 

 have been marshalled by supporters and opponents alike, 

 it will be of interest to point out the novel aspects 

 and lines of reasoning which have come into prominence 

 through the voluminous discussion belonging to this 

 subject. They were prepared before the appearance of 

 Weismann's writings by the changed and enlarged con- 

 ceptions which the discoveries of the middle of the 

 century introduced concerning the general phenomena 

 of Life, Death, and Disease. Three distinct convictions 

 regarding these three main aspects of the living portion 

 of creation have been forced upon the scientific and 

 popular mind. First, we have the modern doctrine of 

 the ubiquity of organisms and germs, at least so far 

 as our planet is concerned : beyond this sphere we 

 can say that we know no more of the existence 

 of living matter than past generations. Secondly, we 

 have the generally recognised doctrine that spontan- 

 eous generation of living out of not-living matter is 

 unknown and inconceivable under such conditions as 



