330 THEORIES OF INHERITANCE AND DEVELOPMENT 



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 play of physical forces upon it 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 limit 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 repress the hope 

 that step by step the way may yet be opened to an understanding of 

 inheritance and development. 



LITERATURE. IX 



Boveri, Th. Ein geschlechtlich erzeugter Organismus ohne mutterliche Eigen- 



schaften: Sitz.-Ber. d. Ges. f. Morph. und Phys. in Miinchen, V. 1889. See 



also Arch . f. Entwm . 1 895 . 



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



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

 Herbst, C. Uber die Bedeutung der Reizphysiologie fur die kausale Auffassung 



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



1894-95. 



Hertwig, 0. Altere und neuere Entwickhmgs-theorieen. Berlin, 1892. 

 Id. Urmund und Spina Bifida : Arch. mik. Anat., XXXIX. 1892. 

 Id. Uber den Werth der ersten Furchungszellen fur die Organbildung des 



Embryo : Arch, mik. Anat., XLII. 1893. 

 Id. Zeit und Streitfragen der Biologic. Berlin, 1894. 

 His, W. Unsere Korperform und das physiologische Problem ihrer Entstehung. 



Leipzig, 1874. 

 Loeb, J. Untersuchungen zur physiologischen Morphologic: I. Heteromorphosis. 



Wurzburg, \%<)\ . II. Organbildung und Wachsthum. IVurzburg, 1892. 

 Id. Some Facts and Principles of Physiological Morphology: Wood's Holl Biol. 



Lectures. 1893. 

 Nageli, C. Mechanisch-physiologische Theorie der Abstammungslehre. M'un- 



chen u. Leipzig, 1884. 

 Roux, W. Uber die Bedeutung der Kernteilungsfigurcn. Leipzig. 1883. 



