24 
COMPARATIVE ZOOLOGY. 
a similar simple cellular structure; and all animal tissues, 
while forming, are cellular. But this character, which is 
permanent in plants, is generally transitory in animals. 
In the more highly organized tissues the cells are so united 
as partly or wholly to lose their individuality, and the 
characteristic part of the tissue is the intercellular sub¬ 
stance, while the cells themselves are small and unim¬ 
portant, or else the cells are fused together and lose their 
dividing walls, as in striated muscles and in nerves. Ex¬ 
cepting the lowest forms, animals are more composite than 
plants, i. e., their organs are more complex and numerous, 
and more specially devoted to particular purposes. Kep- 
etition of similar parts is a characteristic of plants; and 
when found in animals, as the Angle-worm, is called vege¬ 
tative repetition. Differentiation and specialization are 
characteristic of animals. Most animals, moreover, have 
fore-and-aft polarity; in contrast, plants are up-and-down 
structures, though in this respect they are imitated by 
radiated animals, like the Star-fish. Plants are continually 
receiving additional members; most animals soon become 
perfect. 
(5) Physiology.— In their modes of nutrition, plants and 
animals stand widest apart. A plant in the seed and an 
animal in the egg exist in similar conditions: in both 
cases a mass of organic matter accompanies the germ. 
When this supply of food is exhausted, both seek nourish¬ 
ment from without. But here analogy ends: the green 
plant feeds on mineral matter, the animal on organic. Some 
plants have the power to form chlorophyl, the green color¬ 
ing matter of leaves, which uses the energy of the sunlight 
to form starch out of the inorganic substances—carbon- 
dioxide and water. They are able also to form albuminoid 
matter out of inorganic substances. A very few animals 
which have a substance identical with or allied to chloro¬ 
phyl have the same power, but in general animals are de- 
