INTRODUCTION. 9 



latter, indeed, has asserted that " biology is not the 

 less exact, our conclusions are no less accurate, because 

 they are oialj probably true." 



However this may be, science which investigates 

 causation must to us be based upon likelihood ; and 

 therefore, so far, it partakes of a compromise. The 

 intelligent grouping of facts in one series, is uniformly 

 attended by another smaller series, which will not lend 

 themselves to the conception of the law deducible from 

 the first grouping. But no one will loyally put aside 

 that law, if it be reasonable, on account of exceptions 

 to it ; but receive it as an instalment and vantage- 

 ground gained from the land of ignorance. 



If a master-mind rises afterwards and regroups 

 these facts under another law, so as to eliminate more 

 exceptions, the student who would ascertain the causes 

 of things, will give in his adherence to the new line of 

 thought, even though it be like " Phantasie." The 

 system of Ptolemy thus has given place to that of 

 Copernicus ; phlogiston to the atom of Dalton, which 

 again suffers modification in the modern atomicity of 

 Gerhardt, Graham, Wurtz, or Brodie. 



After all, a knowledge of the fundamental cause of 

 variation is the high desideratum of the biologist. 

 How and why certain cells should have special func- 

 tions of segregation for elaborating different organs we 

 know not, and at present we see but little chance of 

 knowing. Certain cells have been regarded as vehicles 

 containing groups of physiological units, ready to obey 

 their proclivity towards the structural arrangements of 

 the species they belong to. This appears to accord 

 with the views of Herbert Spencer, who assumes 

 " units of protoplasm lower in degree than the visible 

 cell-units of plastids, and which may be regarded as in 

 a polar condition." 



Prof. Ray Lankester remarks that modified force- 

 centres may become further modified in each genera- 

 tion, and this might be made to fit in with Mr. Darwin's 

 theory of Pangenesis. The subject, however, is beset 



