THE OUTER TISSUES OF ANIMALS. 309 



However this may be, the force of the general argument 

 remains the same. In these exchanges of structure and 

 function between the outer and quasi- outer tissues, we get 

 undeniable proof that they are easily differentiable. And 

 seeing this, we are enabled the more clearly to see how there 

 have, in course of time, arisen those extreme and multi- 

 tudinous differentiations of the outer tissues that have been 

 glanced at. 



