No. 589] THE EVOLUTION OF THE CELL 23 



simple or complex in structure, all the parts are equally 

 " living" and equally indispensable for the maintenance 

 of life, or at least for the continuance of the vital func- 

 tions in the normal, specific manner, without losing the 

 right to inquire which of those parts are the phyloge- 

 netically older. A simple analogy will serve to point my 

 meaning. A man could not continue to live for long if 

 deprived either of his brain, his digestive tract, his lungs, 

 his heart, or his kidneys, and each of these organs is both 

 "living" in itself and at the same time an integral part 

 of the entire organization of the human body ; yet no one 

 would think of forbidding comparative anatomists to dis- 

 cuss, from the data at their command, which of these 

 organs appeared earlier, and which later, in the evolution 

 of the phylum Vertebrata. Moreover, speculative though 

 such discussions must necessarily be, there is no one 

 possessing even a first-year student's knowledge of the 

 facts who would controvert the statement that the diges- 

 tive tract of man is phylogenetically older than the lung. 

 Speculative conclusions are not always those that carry 

 the least conviction. 



T|ie evolution of the cell may be discussed as a morpho- 

 logical problem of the same order as that of the phy- 

 logeny of any other class or phylum of living beings, and 

 by the same methods of inquiry. In the first place there 

 is the comparative method, whereby different types of 

 cell-structure can be compared with one another and with 

 organisms in which the cell-structure is imperfectly de- 

 veloped, in order to determine what parts are invariable 

 and essential and what are sporadic in occurrence and 

 of secondary importance, and if possible to arrange the 

 various structural types in one or more evolutionary 

 series. Secondly there is the developmental or ontoge- 

 netic method, the study of the mode and sequence of the 

 formation of the parts of the cell as they come into exist- 

 ence during the life-history of the organism. Both these 

 methods, which are founded mainly on observation, re- 

 quire to be checked and controlled by the experimental 



