weismann's theory of heredity. 435 



To the probable objection that the supposed geinniules must be of 

 impossibly minute size — seeiug that thousands of millions of them 

 would require to be packed into a single ovum or spermatozoon — Mr. 

 Darwin opposes a calculation that a cube of glass or water having 

 only one ten-thousandth of an inch to a side contains somewhere be- 

 tween sixteen and a hundred and thirty-one billions of molecules. 

 Again, as touching the supposed power of multiplication on the part 

 of his gemmules, Mr. Darwin alludes to the fact that infectious mate- 

 rial of all kinds exhibits a ratio of increase quite as great as any that 

 his theory requires to attribute to gemmules. Furthermore, with 

 respect to the elective aflinity of gemmules, he remarks that '' in all 

 ordinary cases of sexual reproduction the male and female elements 

 certainly have an elective aflinity for each other ; " of the ten thousand 

 species of Composita?, for example, " there can be no doubt that if the 

 l)ollen of all these species could be simultaneously placed on the stigma 

 of auy one species, this one would elect, with unerring certainty, its 

 own i)olleu." 



Such then in brief outline, is Mr. Darwin's theory of pangenesis. 



Professor Weismann's theory of germ-plasm is fundamentally based 

 upon the great disti action tliat obtains in respect of their transmissi- 

 bility between characters which are congenital and characters which 

 are acquired. By a congenital character is meant any individual pecul- 

 iarity, whether structural or mental, with which the individual is 

 born. By an acquired character is meant any peculiarity which the 

 individual may subsequently develop in consequence of its own indi- 

 vidual experience. For example, a man may be born with some mal- 

 formation of one of his fingers or he may subsequently acquire such a 

 malformation as the result of accident or disease. Now in the former 

 case — i. e., in that where the malformation is congenital — it is ex- 

 tremely probable that the peculiarity will be transmitted to his chil- 

 dren ; while in the latter case — i. e., where the malformation is subse- 

 quently acquired — it is virtually certain that it will not be transmitted 

 to his children. And this great difference between the transmissibility 

 of characters which are congenital and characters which are acquired 

 extends universally as a general law throughout the vegetable as well 

 as the animal kingdom, and in the province of mental as in that of 

 bodily organization. Of course this general law has always been well 

 known and more or less fully recognized by all modern physiologists 

 and medical men. But before the subject was taken up by Professor 

 Weismann it was generally assumed that the diflerence in question 

 was one of degree, not one of kind. In other words, it was assumed 

 that acquired character?, although not so fully— and therefore not so 

 certainly — inherited as congenital characters, nevertheless were inher- 

 ited in some lesser degree ; so that, if the same character continued to 

 be developed successively in a number of sequent generations, what 

 was at first only a slight tendency to be inherited would become by 



