The fact, then, that it is specific characters which vary most is agreed upon by both Lamarck and Mr. Darwin. Lamarck, however, maintains that it is these specific characters which are most capable of being affected by the habits of the creature, and that it is for this reason they will be most variable, while Mr. Darwin simply says they are most variable, and that, this being so, the favourable variations will be preserved and accumulated—an assertion which Lamarck would certainly not demur to.
"Irregular degrees of perfection," says Lamarck, "and degradation in the less essential organs, are due to the fact that these are more liable than the more essential ones to the influence of external circumstances: these induce corresponding differences in the more outward parts of the animal, and give rise to such considerable and singular difference in species, that instead of being able to arrange them in a direct line of descent, as we can arrange the main groups, these species often form lateral ramifications round about the main groups to which they belong, and in their extreme development are truly isolated."[259]
In his summary of the second chapter of his 'Origin of Species,' Mr. Darwin well confirms this when he says, "In large genera the species are apt to be closely, but unequally, allied together, forming little clusters round other species."
"A longer time," says Lamarck, "and a greater influence of surrounding conditions, is necessary in order to modify interior organs. Nevertheless we see that Nature does pass from one system to another without any sudden leap, when circumstances require it, provided the systems are not too far apart. Her method is to proceed from the more simple to the more complex.[260]
"She does this not only in the race, but in the individual." Here Lamarck, like Dr. Erasmus Darwin, shows his perception of the importance of embryology in throwing light on the affinities of animals—as since more fully insisted on by the author of the 'Vestiges of Creation,' and by Mr. Darwin,[261] as well as by other writers. "Breathing through gills is nearer to breathing through lungs than breathing through trachea is. Not only do we see Nature pass from gills to lungs in families which are not too far apart, as may be seen by considering the case of fishes and reptiles; but she does so during the existence of a single individual, which may successively make use both of the one and of the other system. The frog while yet a tadpole breathes through gills; on becoming a frog it breathes through lungs; but we cannot find that Nature in any case passes from trachea to lungs."[262]
Lamarck now rapidly reviews previous classifications, and propounds his own, which stands thus:—I. Vertebrata, consisting of Mammals, Birds, Fishes, and Reptiles. II. Invertebrata, consisting of Molluscs, Centipedes, Annelids, Crustacea, Arachnids, Insects, Worms, Radiata, Polyps, Infusoria.
"The degradation of organism," he concludes, "in this descending scale is not perfectly even, and cannot be made so by any classification, nevertheless there is such evidence of sustained degradation in the principal groups as must point in the direction of some underlying general principle."[263]
Lamarck's sixth chapter is headed "Degradation and Simplification of the Animal Chain as we proceed downwards from the most complex to the most simple Organisms."
"This is a positive fact, and results from the operation of a constant law of nature; but a disturbing cause, which can be easily recognized, varies the regular operation of the law from one end to the other of the chain of life.[264]
"We can see, nevertheless, that special organs become more and more simple the lower we descend; that they become changed, impoverished, and attenuated little by little; that they lose their local centres, and finally become definitely annihilated before we reach the lowest extremity of the chain.[265]