72 ON HEREDITY, [H. 



itself, while its connection with the nearest nerve-cell can be 

 frequently traced, and the function of many groups of cell 

 elements is already known. But it would seem to be impos- 

 sible to unravel the excessively complex network into which 

 the cells and fibres are knit together ; and hence to arrive at 

 the function of each single element appears to be also 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 

 investigation 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 

 property of an organism by which its peculiar nature is trans- 

 mitted to its descendants. From an eagle's ^gg 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 overgrowth 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 interpretation 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 cer- 

 tain 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 



