228 GEOLOGICAL BIOLOGY. 



differ also in their structural relation to the whole organism 

 of which they are parts. Homology may be said to be based 

 upon morphological unity, and analogy is based upon func- 

 tional or physiological unity. 



Differentiation Illustrated in the Case of Motor Organs To 

 illustrate this mode of analysis of the organic structure from 

 the point of view of extent of differentiation of parts, or in 

 order from homogeneity to heterogeneity of structure, a 

 study may be made of the devices developed for the execu- 

 tion of motion or locomotion, in the various branches of the 

 Animal Kingdom. 



Organic motion, in its simplest form, is contraction, the 

 bringing together of two ends of a contractile' tissue, as 

 muscle, with no hard parts, no specialized organs : this is what 

 is seen in the lowest forms of the Protozoa, and expresses 

 itself in change of form of a globule, drawing in of a part, or 

 pushing out of another part. 



Two Directions in which Differentiation Proceeds In dif- 

 ferentiating the mechanism of motion, elaboration may take 

 place in two directions. 



(A) By subdivision, or multiplication of the moving parts, 

 and increasing the rapidity of the contracting: this results in 

 ciliary motion, and the specialized organs thus elaborated are 

 called cilia. 



(B) The second is by concentration, or massing of the parts 

 of motion, and thus increasing the energy expressed in a 

 single motion : this leads to the construction of muscular 

 tissue, and the expression of specialized muscular motion. 



In (A) the direction of the motion is indefinite, in (B) it is 

 definite in direction and united in time, or period of action. 



Ciliary Motion. The real function of ciliary motion is seen 

 in an augmented state in the special organs called tentacles, 

 which act by muscular methods, but whose function is vibra- 

 tile. These may add the functions of ingestion and prehen- 

 sion to those of simple ciliary motion. But ciliary motion 

 itself is fundamentally applied for the ingestion of food. 

 This is accomplished in minute organisms either by causing 

 the organism itself to move in its medium towards the food, 

 or by setting up currents in the medium and thus causing the 



