14 MORPHOLOGICAL DEVELOPMENT. 



ing a general similarity. Here from the edge of a pool is 

 a reed, and here from the adjacent copse is a hemlock: the 

 one having grown tall in escaping the shade of its com- 

 panions and the other in escaping the shade of the surround- 

 ing brushwood. On being cut across each discloses a tube, 

 and each exhibits septa dividing this tube into chambers. 

 In either case by the tubular structure is gained the greatest 

 strength with the least ihaterial; but there is no morpho- 

 logical kinship between the tubes nor between the septa. 

 Still more marked is the simulation of homology by analogy 

 in another plant which the adjacent ditch may furnish — the 

 common Horsetail. In this, again, we see an elongated ver- 

 tical-growing part, raising the foliage into the air; and, as 

 before, this is tubular and divided by septa. A type utterly 

 alien from the other two has, by survival of the fittest, been 

 similarly moulded to meet mechanical needs. 



Passing now to the obscurations in the animal world 

 caused by alterations favouring locomotion, we note first that 

 the locomotive power is at the outset very slight. Among 

 many orders of Protozoa, as also among many low types of 

 Metazoa, vibratile cilia are the most general agents of loco- 

 motion — necessarily feeble locomotion. Eegarded in the 

 mass, the Cwlenterata, when not stationary like the Hydra or 

 higher types in the hydroid stage, usually possess only such 

 small self-mobility as the slow rhythmical contractions of 

 their umbrella-disks effect, or else such as is effected by bands 

 of cilia or of vibratile plates, as in the Beroe. Even among 

 these low tpes of Metazoa, however, in which ordinarily the 

 radial structure is conspicuous, or but slightly obscured by 

 an ovoid form as in the Ctenophora, we find, in the Cestus 

 veneris, extreme obscuration caused by an elongation which 

 facilitates movement through the water; alike by the actions 

 of its vibratile plates and by its undulations, which simulate 

 those of sundry higher animals. 



And here we come upon the essential fact to be recognized. 

 Elongation favours locomotion in various ways that are 



