SCIENCE IN THE LAST HALF CENTURY. 91 



of science. But, whether the explanation of these extreme variations 

 in the mean temperature of a great part of the northern hemisphere is 

 to be sought in the concomitant changes in the distribution of land and 

 water surfaces of which geology affords evidence, or in astronomical 

 conditions, such as those to which I have referred, is a question which 

 must await its answer from the science of the future. 



BIOLOGY. 



Turning now to the great steps in that vast progress which the bio- 

 logical sciences have made since 1837, we are met, on the threshold of 

 our epoch, with perhaps the greatest of all, — namely, the promulgation 

 by Schwann, in 1839, of the generalization known as the " cell theory," 

 the application and extension of which by a host of subsequent investi- 

 gators has revolutionized morphology, development, and physiology. 

 Thanks to the immense series of labors thus inaugurated, the following 

 fundamental truths have been established: 



All living bodies contain substances of closely similar physical and 

 chemical composition, which constitute the i)hysical basis of life, known 

 as protoplasm. So far as our present knowledge goes, this takes its 

 origin only from preexisting i^rotoplasm. 



All complex living bodies consist, at one period of their existence, of 

 an aggregate of minute portions of such substance, of similar structure, 

 called cells, each cell having its own life independent of the others, 

 though influenced by them. 



All the morphological characters of animals and plants are the results 

 of the mode of multiplication, growth, and structural metamorphosis of 

 these cells, considered as morphological units. 



All the physiological activities of animals and plants — assimilation, 

 secretion, excretion, motion, generation — are the expression of the ac- 

 tivities of the cells considered as physiological units. Each individual, 

 among the higher animals and plants, is a synthesis of millions of sub- 

 ordinate individualities. Its individuality, therefore, is that of a " civ- 

 itas" in the ancient sense, or that of the Leviathan of Hobbes. 



There is no absolute line of demarkation between animals and plants. 

 The intimate structure, and the modes of change, in the cells of the two 

 are fundamentally the same. Moreover, the higher forms are evolved 

 from lower, in the course of their development, by analogous processes 

 of differentiation, coalescence, and reduction in both the vegetable and 

 the animal worlds. 



At the present time the cell theory, in consequence of recent inves- 

 tigations into the structure and metamorphosis of the " nucleus," is 

 undergoing a new development of great significance, which, among 

 other things, foreshadows the possibility^ of the establishment of a phy- 

 sical theory of heredity, on a safer foundation than those which Butfon 

 and Darwin have devised. 



