176 Elementary Zoology. 



regular series of chambers, a feature which formerly absolutely 

 separated all Oligchseta from all Hirudinea. The first proper 

 description of Peripatus broke down a number of the bounda- 

 ries which kept the annelids apart from the arthropods ; and 

 many similar instances might be cited. Facts like these — and 

 they are multiplying every day — show that classification, if 

 possible practically, is theoretically impossible ; the paradox, in 

 fact, is true, that the less perfect our knowledge, the more 

 complete our schemes of classification ; the existence of clear 

 classifications is an expression of ignorance. They also lead to 

 the inference that there has been, and is still, a gradual evolu- 

 tion of forms of life. Could we have before us all the forms 

 that exist and have existed (it is important to bear in mind the 

 immense numbers of totally extinct creatures, mostly only 

 represented by often unintelligible fragments), it might be im- 

 possible to define a line separating man from the amoeba. 



Animals and Plants. 



More than this, it is not possible to draw a clear line 

 between plants and animals. The only line that can at 

 present be absolutely drawn is between living creatures and 

 minerals. To divide nature into Organisata and Inorgani- 

 sata is really the only scheme of classification that can be 

 fully proved. 



As to animals and plants, it may be useful to point out 

 some of the more salient features by which they are separated. 

 But it will conduce towards clearness if, first of all, their 

 essential similariiies are enumerated. Plants, Uke animals, are 

 built up of cells, or else exist as single cells. The cells in 

 both consist essentially of a mass of protoplasm, of similar 

 composition, enclosing a nucleus (or nuclei). 



The nucleus, in dividing, may or may not, in both, undergo 

 that elaborate series of changes collectively termed karyoki- 

 nesis (see p. 129). 



All multicellular plants, like multicellular animals, begin 

 life as a single cell, the ovum or oosphere. 



