ORIGIN AND SIGNIFICANCE OF SPINES 27 



was remarked at the beginning of this paper, that single 

 causes were not sufficient in every case to account for spine 

 growth, and while it is comparatively easy to formulate 

 abstract expressions or terms covering all possible cases, it 

 will be found difficult to construe properly certain factors to 

 fit into an}' particular conception. In illustration of this, 

 the foregoing statements may be taken. Thus spines are 

 formed by the only means possible, either by growth of new 

 tissue or by decrease in old. Again, the forces must act 

 from the interior or from the exterior ; in other words, they 

 must be intrinsic or extrinsic. But in some specific instance, 

 while considering food, forces of nutrition, external or in- 

 ternal demands, reactions, etc., a question may arise as to 

 the proper disposition to make of a spine developing primarily 

 by external stimuli and becoming a defence and secondarily 

 a weapon ; yet which by differentiation in time loses some of 

 its protective and offensive qualities, and by selection may be 

 confined to one sex. 



Growth and decline are underlain by the processes taking 

 place in individual cells as well as in aggregates of cells, for 

 spine growth must be considered in unicellular as well as 

 multicellular organisms. 



Ryder^i has very philosophically discussed the correlations 

 of volumes and surfaces of organisms, and has reached the 

 conclusion that " the physiological function of a cell is also a 

 function of its figure, i. e., of its morphological character ; 

 that is to say, cells tend to elongate in the direction of the 

 exercise of their function." Out of this may be deduced the 

 correlative conclusion that aggregates of cells having a like 

 function also tend to elongate in the direction of the exercise 

 of this function ; and, further, it may be asserted that parts 

 or portions of cells will act in the same manner. 



A familiar illustration of these principles as applied to a 

 single cell may be taken from the rhizopod Amoeba proteus. 

 When disturbed by incident forces in all directions, it assumes 

 a globular form. Under continuous motion of its own, it is 

 elongated in the axis of motion, its larger pseudopodia being 



