34 OUTLINES OF EVOLUTIONARY BIOLOGY 



members of the animal and vegetable kingdoms we can point to 

 many obvious distinctions. The higher plants are fixed and 

 stationary, while the animals move about from place to place by 

 means of special organs of locomotion. The animals have 

 complex digestive, respiratory, excretory, nervous and sensory 

 organs, which are wanting in the plants. Lastly, the animals 

 have no chlorophyll and cannot therefore, like the green plants, 

 obtain their supplies of energy directly from the sun's rays by 

 photosynthesis, but must depend upon the potential energy con- 

 tained in the complex molecules of their food, which they obtain 

 ready made from the bodies of other organisms. 



Amongst the lower organisms, however, we find that most of 

 these distinctions disappear. Thus many of the lower plants 

 move about actively while many of the lower animals, such as 

 the sponges, hydroids and corals, are fixed and stationary in the 

 adult condition, though still showing their animal nature in other 

 respects, such as their method of nutrition. This mixture of 

 what were once regarded as distinctively animal and vegetable 

 characters in such forms as the corals and hydroids gave rise to 

 the name " zoophytes," or animal-plants, by which these organisms 

 were known to the older naturalists. 



When we descend to forms still lower in the scale of organiza- 

 tion, consisting each of a single cell, we find that every dis- 

 tinction may disappear except that of the presence or absence of 

 chlorophyll and the mode of nutrition immediately dependent 

 thereon : plant-like or holophytic as in Hsematococcus, animal- 

 like or holozoic as in Amoeba. 1 



It cannot be maintained, however, that even these characters 

 form an absolute distinction between plants and animals, for, in 

 the first place, many undoubted plants, such as the Fungi, have 

 lost their chlorophyll by degeneration, and, in the second place, 

 while botanists claim Haematococcus and the forms closely related 

 to it as plants, zoologists claim them as animals, chiefly because 

 they are so closely related in structure to other unicellular 

 flagellates which contain no chlorophyll that we cannot refuse to 

 include them in the same group. 



1 The nutrition of any typical green plant is holophytic, that of any typical 

 animal holozoic. The latter term implies the taking in of solid food derived from 

 the bodies of other organisms, and is thus distinguished from the saprophytic type 

 of nutrition met with in many of the lower animals and plants (e.g. Fungi), which 

 consists in the absorption of liquid food derived from the decaying bodies of other 

 organisms. 



