MASON : VARIATION IN THE SHELLS OF THE MOLLUSCA. 329 



exactly alike. Sometimes varieties, acknowledged to belong to 

 the same species, differ much more among themselves than do 

 good species from each other. The distinction generally 

 accepted between species and varieties is that the hybrids 

 between different varieties of the same species are fertile, while 

 the mules resulting from the union of different species are 

 infertile. 



Granting the existence of species, the conception of a type 

 is essential. In this connection the word type is used in two 

 different senses. 



Strictly speaking, the type is the specimen first described 

 by the original author, but in many cases the species was first 

 described from an aberrant specimen, especially where a de- 

 scriber yielded to the temptation of founding a new species on a 

 single individual, a proceeding always dangerous. The meaning, 

 however, in which I shall use the word this evening is something 

 different, viz., the average individual. 



It is a curious fact that the first classification of Animal 

 Variations ever formulated was published by Professor A. 

 Brandt at the last International Congress of Zoology. He 

 divides them into three categories, viz., I. Spontaneous ; 

 II. Acquired ; III. Hereditary. 



I. The Spontaneous may be (a) strictly individual ; (/3) proper 



to the vital cycle ; (y) proper to the sex; (S) proper to 

 future generations. 



II. The Acquired may be due to (a) external action, mechani- 

 cal, physical, chemical, or composite ; (/?) due to function, 

 such as use or disuse ; (y) due to pathological processes ; 

 (S) or correlated with the preceding variations. 



III. Hereditary (a) directly from parents ; (13) indirectly by 

 influence; (y) by atavism from more remote ancestors. — 

 Joiirn. R. Micr. Soc, 1893. 



The formation of new species and varieties by natural 

 selection, or the survival of the fittest, by the gradual accumula- 



