200 CONK LIN. [Vol. XIII. 



importance in building the adult ; among these may be men- 

 tioned the early appearance of bilateral symmetry; the appear- 

 ance of bilateral symmetry in diverse directions in the different 

 layers always associated with the future rotation of some of 

 the layers in a definite direction ; the segregation of materials 

 for certain organs, layers, and regions of the body into definite 

 cells ; the distribution of yolk to the various blastomeres, 

 being found in some cases in many cells, in others being 

 largely confined to a single cell ; all these and a hundred other 

 determinate characters have only a prospective value and must 

 have been produced, either directly or indirectly, by the influ- 

 ence of the later upon the earlier stages. 



(2) Precocious differentiation, while indicating a shortening 

 of the process of development, does not indicate a shortening in 

 its duration. Many animals of high organization run through 

 their development in a very short time and yet show no traces 

 of precocity, while many lower animals, although showing a 

 high degree of precocity, yet develop very slowly; e.g., the 

 chick develops in twenty-one days, Crepidula reaches its larval 

 stage only at the end of four weeks, and yet in the former case 

 no precocity is apparent in the early stages, whereas it appears 

 at the very beginning of development in the latter. Even 

 within the limits of a single group the rate of development 

 varies greatly, though apparently the precocity does not ; e.g., 

 the relatively rapid development of pteropods as compared with 

 prosobranchs. 



Numberless instances might be given to show that the 

 rapidity of development does not depend upon the amount of 

 yolk contained in the ^g^, as the text-books always have it, nor 

 upon the temperature at which normal development occurs, 

 but rather upon the individual peculiarities of the protoplasm 

 itself (cf. Kofoid '95 and Castle '96). The shortening of the 

 time of development, therefore, is not in any way correlated 

 with precocious differentiation, and hence it is unwarrantable 

 to assume that the latter has been produced by natural selec- 

 tion, owing to the beneficial effects of the former. 



(3) Precocity does not insure the development of a larger 

 number of individuals, nor does an egg which manifests pre- 



