274 ANIMAL MORPHOLOGY 



certain biologists, convenient formulas which served to mask the 

 ignorance in which we most frequently find ourselves in regard to 

 the immediate cause of variation. 



Nevertheless, when in 1880 he published his very suggestive little 

 book, Die Existenzbedingungen der Thiere, Carl Semper already 

 attempted to lead back the naturalists to the study of primary 

 factors. To the experiments of rare precursors concerning the mor- 

 phogenic influence of change of alimentary regimen (Hunter, Ed- 

 mondstone), of the modifications of salinity of the water (Smanke- 

 vitch), of heat, of light, etc., he added original researches concerning 

 the best conditions for crossing and for the reproduction of Lim- 

 naea, and especially he brought together into a volume which, 

 although limited, was very complete in its content for the period, 

 an enormous mass of biological observations, many of which have 

 exactly the same demonstrative value as the best laboratory ex- 

 periments. Since then, investigations of this sort have been under- 

 taken with enthusiasm on many sides and especially in America. 

 The impulse is given and we may rest assured that the movement 

 will increase in force as the parallel advances of physics and chem- 

 istry permit of the application of greater precision in these studies 

 and of access to certain questions which up to the present seemed 

 inaccessible. 



The opening-up of new scientific fields such as physical chemistry 

 and bio-chemistry will soon furnish us means for taking up success- 

 fully the work which Lamarck was able to trace only in its general 

 outline. 



The dependence of morphology in its relation to the physical 

 and chemical sciences is still more manifest in that branch, so new 

 and so full of promise, which we know under the name of cytology. 



Although the cellular theory, already sketched by Malpighi, 

 had been completely formulated by Raspail (1835) and by Schleiden 

 (1838) for plants, then by Schwann (1839) for animals; although 

 Virchow about the middle of the last century had proclaimed his 

 celebrated aphorism omnis cellula e cellula, it is only during the 

 last twenty years that cellular morphology and cytology have 

 attained a wonderful development, thanks to the investigations 

 of Van Beneden, of Strasburger, and of a brilliant group of young 

 biologists. 



The history of this magnificent structure, its general plan and 

 its details, have been very exactly retraced in a work already class- 

 ical, The Cell in Development and Inheritance, published as early 

 as 1896 by E. B. Wilson, one of the able investigators who with 

 0. and R. Hertwig, Boveri, Maupas, Guignard, etc., have most 

 actively contributed to its construction. But how laboriously this 

 difficult work has been prepared by the numerous improvements 



