372 SCIENTIFIC THOUGHT. 



recent times, owing to improvements in the microscope, 

 the existence of organisms without nuclei has become 

 doubtful. To complicate matters still more, to the 

 nucleus have been added the nucleolus, the vacuoles, the 

 central or pole corpuscles of the cell, &c. It is quite 

 evident from this short reference to the changes which 

 the definition of the morphological unit of living matter 

 has undergone, that no complete and accurate descrip- 

 tion lending itself to measurement and calculation could 

 be based upon it. The conception, useful as it may 

 be, has therefore not permitted of predictions, such as 

 mechanical, physical, and even chemical science, abound 

 3. in. " Has one ever," says Delage, " in a single instance 



Impossi- 

 bility of pre- divined in advance the least of those structures which 



diction. 



the microscope has unveiled ? Has one divined the 

 transverse striation of muscles, the cilia of vibratile 

 epithelia, the prolongations of nerve-cells, the action 

 of the retina or the arcades of Corti, the chromosomes 

 of the nucleus, the centrosome of the cytoplasma ? " l 

 Or, to take an example not from the morphology but 

 from the physiology of organic cellular bodies. It is 

 a very general and a very useful property of cells that 

 they readily absorb substances ; in fact, this property 

 is one of the most valuable aids in microscopic exam- 



1 ' L'Heredite,' &c., p. 746. | only possesses twelve pairs of ribs, 



Prof. Weismann, in his celebrated ; would be found to have thirteen or 



' Essays upon Heredity ' (Engl. fourteen in the embryonic state ; 



transl. by Poulton, &c., p. 255), it has been predicted that, at this 



claims for the theory of descent early period of his existence, he 



that " it has rendered possible the 

 prediction of facts, not indeed with 

 the absolute certainty of calcula- 

 tion, but still with a high degree of 

 probability. It has been predicted 

 that man, who, in the adult state, 



would possess the insignificant 

 remnant of a very small bone in 

 the wrist, the so-called os centrale, 

 which must have existed in the 

 adult condition of his extremely 

 remote ancestors." 



