434 INHERITANCE AND DEVELOPMENT 



ganized bodies. It is true that we may trace in organic nature long 

 and finely graduated series leading upward from the lower to the 

 higher forms, and we must believe that the wonderful adaptive mani- 

 festations of the more complex forms have been derived from simpler 

 conditions through the progressive operation of natural causes. But 

 when all these admissions are made, and when the conserving 

 action of natural selection is in the fullest degree recognized, we can- 

 not close our eyes to two facts : first, that we are utterly ignorant of 

 the manner in which the idioplasm of the germ-cell can so respond 

 to the influence of the environment as to call forth an adaptive 

 variation ; and second, that the study of the cell has on the whole 

 seemed to widen rather than to narrow the enormous gap that sepa- 

 rates even the lowest forms of life from the inorganic world. 



I am well aware that to many such a conclusion may appear reac- 

 tionary or even to involve a renunciation of what has been regarded 

 as the ultimate aim of biology. In reply to such a criticism I can 

 only express my conviction that the magnitude of the problem of 

 development, whether ontogenetic or phylogenetic, has been under- 

 estimated ; and that the progress of science is retarded rather than 

 advanced by a premature attack upon its ultimate problems. Yet 

 the splendid achievements of cell-research in the past twenty years 

 stand as the promise of its possibilities for the future, and we need 

 set no Umit to its advance. To Schleiden and Schwann the present 

 standpoint of the cell-theory might well have seemed unattainable. 

 We cannot foretell its future triumphs, nor can we doubt that the 

 way has already been opened to better understanding of inheritance 

 and development. 



LITERATURE. IX 



Barfurth, D. — Regeneration und Involution: Merkel n. Bonnet, Ergeb., I.-VIIL 

 1891-99. 



Boveri, Th. — Ein geschlechtlich erzeugter Organismus ohne miitterliche Eigen- 

 schaften : Sitz.-Ber. d. Ges.f. MorpJi. inid F/iys. in Aliinchen, V. 1889. See 

 also Arch. f. Entiv. 1895. 



Brooks, W. K. — The Law of Heredity. Baltimore. 1883. 



Id. — The Foundations of Zoology. New York. 1899. 



Davenport, C. B. — Experimental Morphology: I., II. New York. 1897, 1899. 



Driesch, H. — Analytische Theorie der organischen Entwicklung. Leipzig. 1894. 



Id. — Die Localisation morphogenetischer Vorgange: .4rc/i. Entw.,Y\\. i. 1899. 



Id. — Resultate und Probleme der Entwickelungs-physiologie der Tiere : Merkel it- 

 Bonnet. Ergel)., VIII., 1898. (F\ill literature.) 



Herbst, C — Uber die Bedeutung der Reizphysiologie flir die kausale Auffassung 

 von Vorgangen in der tierischen Ontogenese : Biol. Centralh.. XIV., XV. 

 1894-95. 



Hertwig, 0. — Altere und neuere Entvvicklungs-theorien. Berlin. 1892. 



