CONCLUSIONS 39 



is able to live for any length of time by itself. Unless por- 

 tions of both nucleus and cytoplasm are present, the piece 

 separated from the organism dies. Both the nucleus and 

 the cytoplasm take part in the metabolism of the cell, and 

 both are indispensable. If metabolism (Stoffwechsel) be 

 taken in its most generalised sense as the expression of the 

 sum of the phenomena of life, the inherited characters must, 

 in a similar sense, be held as the expression of metabolism. 

 According to Verworn, then, each organism inherits the indi- 

 vidual or characteristic form of metabolism. Cytoplasm and 

 nucleus are both bearers of hereditary substance, and in- 

 heritance can only occur when both are transmitted to the 

 offspring. 



When the available evidence is considered, we are forced [ 

 to the conclusion that nothing like a demonstration as to the ; 

 nature or situation in the cell of the substance that carries \ 

 the common hereditary qualities has yet been made. There ' 

 are many opinions, several worthy of consideration, and many 

 of them forcibly expressed. It is necessary to realise very 

 definitely, however, that we are dealing with personal opinions, 

 and that all of them are not only open to question, but are 

 disputed by one group or other of reputable biologists. 



The only generalisation it is safe to make, is to go a very 

 little further than Huxley did more than thirty years ago. 

 Instead of saying with him it is " probable," we can say it 

 is practically certain that every part of the adult contains 

 molecules derived from the male and from the female 

 parent. 



The more detailed conclusions to be derived from the 

 evidence we have considered may be summarised as follows : 



1. The only origin of cells known to us is from pre-existing 

 cells. 



2. A new multicellular individual is usually produced 

 from the fusion of two cells derived from two similar multi- 

 cellular individuals. 1 



3. The potentiality of producing all the characters of the 



1 The exceptions to this rule are dealt with on pp. 28, 29, 30. 



