438 weismann's theory of heredity. 



reach iiiaturity or first become capal)lo of procreation. Tliis general 

 correlation however is somewhat modified by the time during which 

 progeny are dependent upon their parents for support and protection. 

 Nevertheless, it is evident that this modification tends rather to confirm 

 the view that expectation of life on the part of individuals has in all 

 cases been determined with strict reference to the requirements of prop- 

 agation, if under propagation we include the rearing as well as the 

 production of offspring. I may observe in passing that I do not thiidt 

 this general law can be found to apply to plants in nearly so close a 

 manner as Weismann has shown it to apply to animals ; but leaving 

 this fact aside, to the best of my judgment it does appear that Weis- 

 mann has made out a good case in favor of such a general law with 

 regard to animals. 



We have come then to these results. Protoplasm was originally 

 immortal (barring accidents), and it still continues to be immortal in 

 the case of unicellular organisms which propagate a-sexually. But in 

 the case of all multicellular organisms, which propagate sexually, nat- 

 ural selection has reduced the term of life within the smallest limits 

 that in each given case are compatible with the performance of the 

 sexual act and the subsequent rearing of progeny, reserving however 

 the original endowment of immortality for the germinal elements, 

 whereby a continuum of life has been secured from the earliest appear- 

 ance of life until the present day. 



Now in view of these results, the question arises, Why should the 

 sexual methods of propagation have become so general if their effect 

 has been that of determining the necessary death of all individuals 

 presenting them? Why, in the course of organic evolution, should 

 these newer methods have been imposed on all the higher organisms, 

 when the consequence is that all these higher organisms must pay for 

 the innovation with their lives? Weismann's answer to this question 

 is as interesting and ingenious as all that has gone before. Seeing that 

 sexual propagation is so general as to be practically universal among 

 multi-cellular organisms, it is obvious that in some way or other it 

 must have a most important part to play in the general scheme of 

 organic evolution. What then is the part that it does play? What 

 is its raison (Vetre f Briefly, according to Weismann, its function is 

 that of furnishing congenital variations to the ever- watchful agency of 

 natural selection, in order that natural selection may always preserve 

 the most favorable and pass them on to the next generation by hered- 

 ity. That sexual propagation is well calculated to furnish congenital 

 variations may easily be rendered apparent. We have only to remem- 

 ber that at each union there is a mixture of two germinal elements; 

 that each of these was in turn the product of two other germinal ele- 

 ments in the preceding generation, and so backwards ad injinituin in 

 geometrical ratio. Kemembering this, it follows that the germinal ele- 

 ment of no one member of a species can ever be the same as that of any 



