WEISMANN'S THEORY OF THE GERMPLASM 67 



Formerly, indeed, many biologists, relying upon 

 the optical appearances presented in microscopical 

 investigation, have been inclined to the view that 

 the visible qualities of a tissue, as revealed by the 

 microscope, were the only, or the chief, distinctive 

 characters. For instance, by microscopical investi- 

 gation one cannot distinguish the tendons, nerves, 

 bones, and cartilages of a dog from the corre- 

 sponding tissues in a horse. So far as their special 

 use in the organism goes, one might interchange 

 the corresponding parts in these two mammals. A 

 tendon from the dog, if large enough, might be 

 attached to the muscle of a horse, and would 

 transmit the pull of the muscle on the bone just as 

 well, and would completely satisfy the mechanical 

 duties of the horse's tendon. The same might 

 happen in the case of a bone, of a cartilage, or of a 

 nerve-fibre. 



As a matter of fact, the idea that parts of the 

 tissues of different animals may serve to replace 

 one another has been employed repeatedly in 

 science, especially in the science of medicine. But 

 I believe that our ideas are not yet clear upon the 

 matter. The erroneous impression to which I have 

 alluded has arisen because we do not bear in mind 

 that each tissue, each part of an organ, each cell, 

 possesses, in addition to its obvious characters, very 

 many characters that are invisible to us. Such 

 characters are inherent in the tissue-cells because 

 these are parts of a definite organism. In conse- 

 quence of their specific tissue characters, which are 

 visible to us, we assign cells their place in histo- 



