364 • Zoological Classification. 



though the inflected angles of the jaw would have been obvious 

 enough. And so, though practically any one who met with a 

 characteristically mammalian jaw would be justified in expect- 

 ing to find the characteristically mammalian occipiit associated 

 with it, yet he would be a bold man, indeed, who should strictly 

 assert the belief which is implied in this expectation, viz., that 

 at no period of the world'' s history did animals exist which 

 combined a mammalian occiput with a reptilian jaw, or vice 

 versa." 



Classification may be either natural or artificial ; that is to 

 say, it may be founded upon considerations of wide and endur- 

 ing import, or upon others of a trivial and accidental kind. 

 ".For a classification to be natural and rational, it should 

 include, under the same head, objects which have some dis- 

 tinctive property or structure in common, which is not pos- 

 sessed by other objects, and which is important as well as 

 characteristic. A linear arrangement, in which every object 

 that is placed a degree higher or lower in the scale is really 

 superior or inferior in capacities or development, is impossible, 

 because nature does not work in this simple mechanical way. 

 Nevertheless, in attempting to classify the animal kingdom, it 

 is most natural to begin at one end or the other, either with 

 the most important members of the mammalian group, or with 

 the simplest organisms in which the lowest kind of animal 

 life can be traced. It is curious to note the connection between 

 size and development. For example, the lowest forms are all 

 small, and the highest that we are acquainted with very far 

 from being the biggest in dimensions. The simplest known 

 animals, the Gregarinida, c< are all microscopic, and any one 

 of them, leaving minor modifications aside, maybe said to con- 

 sist of a sac, composed of a more or .less structureless not 

 very well defined membrane, containing a soft semi-fluid sub- 

 stance, in the midst or at one end of which lies a delicate 

 vesicle; in the centre of the latter a more solid particle.'" 

 Professor Huxley appends to this description the obvious, but 

 highly important reflection, that its statements are all true 

 concerning the ova of any of the animals much higher in the 

 scale. The Gregarinida inhabit the bodies of other animals, 

 and they multiply by becoming encysted and dividing into a 

 multitude of minute objects, called pseudo-navicclkv, from their 

 resemblance in shape to the ship-like diatoms (Naviru/n). 

 When a young pseudo-navicella escapes, it behaves somewhat 

 like an amoeba, and if lucky enough to get swallowed by an 

 appropriate host, it grows into the parent form. The whole 

 life-history of these creatures is not known, as they have not 

 been traced into the exhibition of sexual properties ; and it is 

 therefore possible that their position in the scale may not be 



