122 THE EVOLUTION OF SEX. 



skeletal tissue ; while the white blood corpuscles would be at 

 once recognised as amcebas. Extended observation here also 

 would show him the cells passing from one phase to another. 

 His rough classification of the Protozoa would be verified in the 

 histology of higher animals, and would reappear in the study of 

 their diseases. He would be thus at length in a position to 

 say, that however these three phases were brought about, the 

 forms characteristic of them were of such wide occurrence 

 through nature as to justify his restatement of the familiar cell 

 theory in terms of a larger conception, that of the cell-cycle ; 

 that is to say, from the conception of the cell as a unit mass 

 of living protoplasm, amceboid, encysted, or ciliated, as the 

 case might be, he would come to regard these forms as the 

 predominant phases of a cycle, — primeval, certainly, in the 

 history of the organic world, and largely so even in the 

 individual cell. 



All this time, however, our student has remained a mor- 

 phologist, his use of terms, like active and passive, simply 

 expressing change of place. Not on this path of structural 

 observation alone is it possible to understand what the forms 

 and phases of cells really mean. A final corroboration of the 

 cell-cycle, and at the same time a rationale of it, is obtainable 

 only on physiological lines, when we begin to inquire into the 

 protoplasmic processes which lie behind any change in the 

 form and habit of a cell. We have already spoken of the 

 modern physiologist's conception of living matter, or proto- 

 plasm, as an exceedingly complex and unstable substance or 

 mixture of substances, undergoing continual chemical change 

 or metaboHsm. On the one hand, it is being continually 

 reconstructed by an income of nutritive material, which, at 

 first more or less simple, is worked up by a series of chemical 

 changes till it reaches the chmax of complexity and instability. 

 These upbuilding, constructive, synthetic processes are summed 

 up in the phrase anabolism. But, on the other hand, the proto- 

 plasm is continually, as it "lives,"' breaking down into more and 

 more stable compounds, and finally into waste products. There 

 is a disruptive, descending series of chemical changes known as 

 katabolism. Both constructive and disruptive changes occur in 

 manifold series. The same summit (see p. 89) may be gained or 

 left by many different paths, but at the same time there is, as it 

 were, a distinct watershed, — any change in the cell must tend to 

 throw the preponderance towards one side or the other. In a 



