LESSON XIII 



SPECIES AND THEIR ORIGIN THE PRINCIPLES OF 



CLASSIFICATION 



MORE than once in the course of the foregoing lessons we 

 have had occasion to use the word species for instance, in 

 Lesson I. (p. 8) it was stated that there were different 

 kinds or species of Amoebae, distinguished by the characters 

 of their pseudopods, the structure of their nuclei, &c. 



We must now consider a little more in detail what we 

 mean by a species, and, as in all matters of this sort, the 

 study of concrete examples is the best aid to the formation 

 of clear conceptions, we will take, by way of illustration, 

 some of the various species of Zoothamnium. 



The kind described in the previous lesson is called 

 Zoothamnium arbuscula. As Fig. 26, A, shows, it consists of a 

 tolerably stout main stem, from the distal end of which 

 spring a number of slender branches diverging in a brush- 

 like manner, and bearing on short secondary branchlets the 

 separate individuals of the colony : these are of two kinds, 

 bell-shaped nutritive zooids, and globular reproductive 

 zooids, so that the colony is dimorphic. 



Zoothamnium (or, for the sake of brevity, Z.) alternans 

 (Fig. 27, A) is found also in sea-water, and differs markedly 



