I08 MARINE BIOLOGICAL LABORATORY. 



generative glands, and being thus transmitted to the 

 young, seemed too cumbersome and complex for general 

 acceptance. 



Quite recently Professor Weismann of Freiburg has 

 advanced a theory of Heredity which seems by far the 

 best hitherto offered. This theory is the culmination of 

 a train of thought which he has put forward in essays 

 from time to time during the last few years. These 

 show the gradual growth and development of the theory 

 in the mind of the author, and though some of the facts 

 from which he argues may be open to dispute, yet the 

 ideas which he suggests are so interesting that they 

 are entitled to consideration, even though subordinate 

 to the main plan of his theory. 



In one of his earlier essays he points out that the 

 manner of reproduction among the Protozoa is such 

 that death does not normally occur in this group, for the 

 animal reproduces by merely dividing itself into halves. 

 Thus an adult animal ceases to exist as such, by be- 

 coming: two animals instead of one. It does not die 

 during this process, for there is no corpse, but the 

 whole animal as such has completely disappeared, and 

 in its place we find two individuals so similar that it 

 is impossible to regard them as parent and offspring. 

 Indeed, they cannot be parent and offspring, for they 

 are of the same generation, — it is more natural to call 

 them twins. They are both young animals, for they 

 increase in size, and when adult each of them ceases 

 to exist by dividing itself into two new young ones, and 

 so on indefinitely. 



Hence it would appear that the life-history of such 

 an animal may be di\ided into two periods, — youth and 



