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We are quite unable to resist the impulse of mere vascular and nervous reaction, the watering of the mouth, the jerk of the limb, the closing of the eye which we share with some of the simplest vertebrates. We can only with difficulty resist the instincts of sex and food, of anger and fear, which we share with the higher animals.
The most signal differences in kind are wont to arise when organs originally developed for a certain purpose come to be applied to a very different purpose, as that change of the fish's air-bladder into a lung which accompanied the first development of land vertebrates.
Our next lesson will be entitled "The Ascent of Man," in which the rise of man that is, his body from the lowly forms of the vertebrates is shown. In the same lesson we shall begin our consideration of the "evolution of souls." We trust that the students are carefully studying the details of each lesson, for every lesson has its part in the grand whole of the Teachings.
Where the number of cells formed is approximately similar, as in two allied forms of plant in this case, the greater size of the cells would naturally give a stouter habit, but it is clear that large cells do not necessarily mean greater size. The cells of Salamander and Proteus are the largest found among Vertebrates, but those Amphibia are not the largest Vertebrates.
It can only be said positively that the peculiar ontogeny of the complicated optic apparatus in man follows just the same laws as in all the other Vertebrates. Their eye is a part of the fore brain, which has grown forward towards the skin, not an original cutaneous sense-organ, as in the Invertebrates.
It loses its purely physical character, it ceases to be simply instinctive, it becomes reasoned. With the higher vertebrates it is periodical, or is resorted to for the satisfaction of a given want propagation of the species, migration, hunting, or mutual defence.
We may therefore say that neither form nor complication of structure distinguishes classes, but simply the mode of execution of a plan. In Vertebrates, for instance, how do we distinguish the class of Mammalia from the other classes of the type?
But along with these embryonic characters, which were common to all Devonian fishes, there were other structures in certain groups which foreshadowed the higher structures of the land vertebrates which were yet to come: air sacs which were to develop into lungs, and cartilaginous axes in the side fins which were a prophecy of limbs.
Some of the amphibious animals possess both lungs and gills, even when matured; but the higher vertebrates living in the water breathe through lungs which are evolved from the air-bladder of fishes, which in turn have been evolved from the primitive gullet of the lower forms. There are fishes known which are warm-blooded.
MAMMALS. So far as the entries upon the geological record show, mammals made their advent in a very humble way during the Trias. These earliest of vertebrates which suckle their young were no bigger than young kittens, and their strong affinities with the theromorphs suggest that their ancestors are to be found among some generalized types of that order of reptiles.
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