48 MORPHOLOGICAL DEVELOPMENT. 



as tney become integrated and definitely limited, aggregated 

 acquire the habit of budding out other aggregates, on reach 

 ing certain stages of growth. Cells produce other cells 

 endogenously or exogenously ; and fronds give origin to 

 other fronds from their edges or surfaces. We have seen, too, 

 that the new aggregates so produced, whether of the first 

 order or the second order, may either separate or remain 

 connected. Fissiparously-multiplying cells in some cases 

 fly asunder, while in other cases they unite into threads or 

 laminae or masses ; and fronds originating proliferously from 

 other fronds, sometimes when mature disconnect themselves 

 from their parents, and sometimes continue attached to them. 

 Whether they do or do not part, is clearly determined by 

 their nutrition. If the conditions are such that they can 

 severally thrive better by separating after a certain develop 

 ment is reached, it will become their habit then to separate ; 

 since natural selection will favour the propagation of those 

 which separate most nearly at that time. If, conversely, it 

 profits the species for the cells or fronds to continue longer 

 attached, which it can only do if their growth and subse 

 quent powers of multiplication are thereby increased ; it must 

 happen, through the continual survival of the fittest, that 

 longer attachment will become an established characteristic ; 

 and by persistence in this process, permanent attachment 

 will result, when permanent attachment is advantageous. 

 That disunion is really a consequence of relative innu 

 trition, and union a consequence of relative nutrition, 

 is clear, a posteriori. On the one hand, the separation 

 of the new individuals, whether in germs or as developed 

 aggregates, is a decaying aw T ay of the connecting tissue ; 

 and this implies that the connecting tissue has ceased 

 to perform its function as a channel of nutriment. On 

 the other hand, where, as we see among Phoenogams, there 

 is about to take place a separation of new individuals in 

 the shape of germs, at the point where the nutrition is the 

 lowest, a sudden increase of nutrition will cause the impend- 



