V \ 



88 , | | V V)^ BIOLOGICAL LECTURES. 



2. In what manner do they originate in a given cell ? This 

 ultimately resolves itself into the problem of cell-division 

 (Cytomery) on the one hand and cell-fusion on the other. 



3. What are the probable steps in the ancestral history, by 

 which these structures came into existence ? This belongs to 

 the broad question of Cytogcny as understood in its phylogenetic 

 sense. 



In following out these questions more in detail, it is 

 important to bear in mind at the outset, some vital distinctions 

 involved in the use of the term organ, whether we understand 

 it in a purely .physiological, or in a morphological sense. From 

 the purely physiological standpoint, any structure or member 

 which, by its activity, contributes to the general welfare of the 

 whole organism is an organ, whether that structure may have 

 originated primarily in the organism, 'or may have been derived 

 secondarily from an external source. Thus, the chromatophores 1 

 in the leaves of a plant are the organs of assimilation in that 

 plant ; the " gonidia " in the thallus of a lichen arc the organs 

 of a lichen, in a physiological sense as the heart or lung is the 

 organ in the body of a higher organism. 



But from the morphological side of the case it is different. 

 According to the morphological view, every differential 

 organism, every organism composed of organs, can only have 

 originated from a homogeneous stage by tJie differentiation of 

 its parts. To state this in another way, " however complicated 

 one of the higher animals and plants may be, it begins its 

 separate existence under the form of a nucleated cell. This, 

 by division, becomes converted into an aggregate of nucleated 

 cells : the parts of this aggregate,, following, different laws of 

 growth and multiplication, give rise to the rudiments of the 

 organs ; and the parts of these rudiments again take those 

 modes of growth and multiplication and metamorphosis which 

 are needful to convert the rudiment into the perfect structure." 2 



1 The term chromatophore (Schmitz) is here used in a broad sense, including 

 leucoplasts and the various coloring substances in the flower and the fruit, as well 

 as the chrophyll granules in the leaves of a green plant. The term is synonymous 

 with Arthur Meyer's trophoplast and Schimper's plastid. 



2 T. H. Huxley : Article Biology, Encyclopaedia Britannica, Qth edition, Vol. Ill, 

 p. 682, 1878. 



