INTRA-SELECTION OE SELECTION AMONG TISSUES 247 



others, and multiply more quickly, thus crowding out those that are 

 not attuned to the appropriate functional stimulus. 



In a similar manner Roux interprets, in the light of the struggle 

 of the parts, the striking adaptations in the course, the branching, 

 and the lumen-formation of the blood-vessels, in the direction of the 

 intersecting connective tissue strands in the tail-fin of the dolphin, 

 in the direction of the fibres in the tympanum, and in many other 

 adaptations in the histological structure of complex tissues. 



In this there is manifestly an important step of progress, for 

 it is obvious that the direction of the bone-lamellse and such like 

 could not have been determined by individual selection, and the same 

 is true in regard to many other histological details. It cannot be dis- 

 puted, however, that there is a kind of selection-process here also, 

 similar to that which we think of, with Darwin and Wallace, as 

 occurring between individual organisms. Just as in the latter, which 

 we shall henceforward call personal selection, variability and inherit- 

 ance lead, in the struggle for existence, to the survival of the fittest, 

 so, in histonal differentiation, the same three factors lead to the 

 victory of what is best suited to the parts of the body in question. 

 The tissues and the parts of the tissues have to distribute and arrange 

 themselves so that each comes to fill the place in which it is most /^ 

 effectively and frequently affected by its specific stimulus, that is, the 

 stimulus in regard to which it is superior to other parts ; but these 

 places are also those the occupation of which by the best re-acting parts 

 makes the whole tissue capable of more effective function, and tliere- 

 fore makes its structure the fittest. Variability^ in this case that 

 of embryonic cells with different primary constituents — must be 

 assumed ; inheritance is implied in the multiplication of the cells by 

 division ; and the * struggle for existence ' here assumes its frequent 

 form of a competition for food and space ; the cells which assimilate 

 more rapidly because of the more frequent functional stimulus 

 increase more rapidly, draw away nourishment from the more slowly- 

 multiplying cells around them, and thus crowd these out to a greater 

 or less extent. 



We might even speak of histonal selection among unicellulars, 

 for it is conceivable that in primitive living substance, such as that of 

 a moneron, there may be minute differences among the vital particles, 

 involving also functional distinctions, which, under the influence of 

 diverse stimuli, may gradually give rise to an increasingly complex 

 differentiation. For the variations in the primary living substance 

 most strongly affected by a particular stimulus would tend to accumu- 

 late at the places most frequently reached by that stimulus, and 



