THE INNER TISSUES OF ANIMALS. 371 



ments — it is impossible to think that natural selection has 

 had anything to do with the power of adjustment thus 

 shown. No survival of individuals in which adaptations 

 of this kind, now in one place and now in another, were 

 better and better effected, could account for acquirement of 

 the ability. Nor can it be supposed that the ability might 

 result from a functionally-produced habit; since it is 

 scarcely conceivable that the number of cases in which indi- 

 viduals profited by it (at first a little and gradually more) 

 could be such (even did they survive) as to affect the 

 constitution of the species. Both of the alleged causes of 

 structural modifications are out of court. It is manifest, 

 too, that the foregoing hypothesis respecting bone-formation 

 yields us not the slightest help. 



But on carefully considering the facts, certain phenomena 

 of profound meaning may strike us. Here, in a part of the 

 body where no such tissues ordinarily exist and to which no 

 such structures are ordinarily appropriate, there arise tissues 

 and structures adapted to the physical circumstances imposed 

 on that part. Out of what do these abnormal but appropriate 

 tissues arise? The substances around — osseous, cartilagin- 

 ous, membranous — consist of differentiated elements too far 

 specialized to allow of transformation. These new tissues, 

 then, must originate from the undifferentiated protoplasm 

 pervading the part. The units of this protoplasm, subject to 

 the actions proper to an articulation, begin to assume the ap- 

 propriate histological traits — are determined by local stimuli 

 to form tissues ordinarily associated with such stimuli. What 

 is the inevitable implication? These units — physiological or 

 constitutional, as we may call them — must have possessed 

 latent potentialities of falling into these special arrangements 

 under stress of such conditions. At one point there arises 

 periosteum and at another ligamentous tissue, while for the 

 shaping of the ends of the bones — here into a rude hinged 

 form and there into a rude ball-and-socket form, according to 

 the habitual movements — there goes on some appropriate de- 



