THE STRUCTURE OF PROTOPLASM. 3 



far more philosophical form, omne vivum e vivo, thus expressin<^ 

 a truth which forms the very foundation of all biological teach- 

 ing at the present day. The development of the cell-theory, 

 long afterwards, enabled Virchow to pronounce the more specific 

 aphorism, otmiis celliila e cellula (1855), — a statement involving 

 the highly interesting conclusion that protoplasm is never formed 

 de novo, but always arises from or through the activity of pre- 

 existing protoplasm differentiated into the form of a cell. Still 

 later a like conclusion was reached with respect to at least one 

 of the structural components of the cell, namely, the nucleus, 

 and the work especially of Flemming and Strasburger justified 

 the saying, omnis nucleus e miclco. Not long afterwards, the 

 researches of Schmitz, Schimper, and others showed that in 

 plant cells some, if not all, forms of plastids (for example, the 

 chlorophyll-bodies) likewise arise by the division of preexisting 

 bodies of the same kind. Thus the law of genetic continuity 

 was gradually extended downwards from the grosser and more 

 obvious characters of the organism into the finer details of its 

 structural elements. Genetic continuity, the origin of like from 

 like, may now safely be regarded as a demonstrated fact in the 

 case of all existing organisms and of all cells ; it hardly falls 

 short of the same degree of certainty as applied to the nucleus ; 

 it is probable in the case of various forms of plastids in plant 

 cells; while the centrosome is now being weighed in the balance 

 with the evidence for the moment apparently accumulating on 

 the negative side. 



Up to this point we have been dealing with matters of 

 observed fact. The next and final step was, however, taken in 

 the region of pure speculation, which had in the mean time been 

 at work building upwards from hypotheses regarding the basic 

 composition of protoplasm. Briicke's suggestion, that the cell 

 might be a congeries of bodies more elementary than itself, 

 found a much fuller expression in Herbert Spencer's theory of 

 physiological units ; but it was Darwin's theory of pangenesis 

 that laid the real basis for what followed in the works of 

 De Vries, Wiesner, Weismann, and Hertwig. The common 

 feature in all these later views is the conception of protoplasm, 

 not as a homogeneous substance or mixture of substances, but 



