72 ON HEREDITY. 



into which the cells and fibres are knit together ; and hence to 

 arrive at the function of each single element appears to be :ilso 

 beyond our reach. We have not however commenced to untie 

 this Gordian knot without some hope of success, for who can say 

 how far human perseverance may be able to penetrate into the 

 mechanism of the brain, and to reveal a connected structure and 

 a common principle in its countless elements? But surely this 

 work will be most materially assisted by the simultaneous in- 

 vestigation of the structure and function of the nervous system in 

 the lower forms of life in the polypes and jelly-fish, worms and 

 Crustacea. In the same way we should not abandon the hope of 

 arriving at a satisfactory knowledge of the processes of heredity, 

 if we consider the simplest processes of the lower animals as well 

 as the more complex processes met with in the higher forms. 



The word heredity in its common acceptation, means that pro- 

 perty of an organism by which its peculiar nature is transmitted to. 

 its descendants. From an eagle's egg an eagle of the same species 

 developes ; and not only are the characteristics of the species 

 transmitted to the following generation, but even the individual 

 peculiarities. The offspring resemble their parents among animals 

 as well as among men. 



On what does this common property of all organisms depend ? 



Hackel was probably the first to describe reproduction as 'an over- 

 growth of the individual,' and he attempted to explain heredity as a 

 simple continuity of growth. This definition might be considered 

 as a play upon words, but it is more than this ; and such an inter- 

 pretation rightly applied, points to the only path which, in my 

 opinion, can lead to the comprehension of heredity. 



Unicellular organisms, such as Rhizopoda and Infusoria, increase 

 by means of fission. Each individual grows to a certain size, and 

 then divides into two parts, which are exactly alike in size and 

 structure, so that it is impossible to decide whether one of them 

 is younger or older than the other. Hence in a certain sense these 

 organisms possess immortality: they can, it is true, be destroyed, 

 but, if protected from a violent death, they would live on in- 

 definitely, and would only from time to time reduce the size of 

 their overgrown bodies by division. Each individual of any such 

 unicellular species living on the earth to-day is far older than man- 

 kind, and is almost as old as life itself. 



