78 ELEMENTS OF BIOLOGY. 



In plants we observe the same specialisation of functions 

 that we have formerly seen in animals, in ascending from the 

 lowest forms to the highest ; but, as in animals also, there is 

 an apparent reversal of this law in some cases as far as the 

 functions of reproduction are concerned. The processes, 

 namely, by which a young Exogen is produced, are appa- 

 rently less complex than those by which many of the Cryp- 

 togams are perpetuated, just as the reproduction of a Verte- 

 brate animal is in one way a simpler matter than that of a 

 Hydroid Zoophyte. In all these cases, as we shall see, the 

 essential part of the process consists in the bringing together 

 of a germ-cell or ovum and a sperm-cell or spermatozoid ; 

 and so far as the process of bringing together is concerned, 

 the complexity is certainly on the side of the lower form. 

 It may be said, also, that there are no essential or funda- 

 mental characters by which the ova and sperm-cells of the 

 higher form can be distinguished from those of the lower. 

 This is one of the cases, however, in which simplicity of 

 anatomical structure must not be confounded with simplicity 

 of function. The sperm-cells of a Vertebrate animal may 

 not to our eyes seem very different from those of one of the 

 Algae ; but unquestionably this can only be because our 

 means of observation are not of such a nature as to disclose 

 to us the subtle but immense differences which must of 

 necessity exist. In all cases, also, it is to be borne in mind 

 that the organs by which the generative elements are elabo- 

 rated are of a more complex description in the higher 

 organism. 



In some of the lower plants, such as the Yeast-plant 

 (fig. 19), all the great physiological functions are carried on 

 by single cells, without the presence of any differentiated 

 internal organs. In such simple plants, also, so far as our 

 means of observation allow us to judge, all the peculiarities 

 which distinguish plants physiologically from animals are as 

 strongly pronounced as they are in the highest vegetables. 

 In such forms, then, as the yeast-plant, we have a single cell 



