Note 5 — Chapter II · THE MORPHOLOGICAL COMPOSITION OF PLANTS.

The great mass of early ancestral types—plant and animal—consisting of soft tissues, have left no remains whatever, and we have no reason to suppose that those which left remains fell within the direct ancestral lines of any existing forms. Contrariwise, we have reason to suppose that they fell within lines of evolution out of which the lines ending in existing forms diverged. We must therefore infer that the difficulties of affiliation which arise if we contemplate divergent types now existing, would not arise if we had before us all the early intermediate types. The Mammalia differ in sundry respects from all other kinds of Vertebrata—Fishes, Reptiles, Birds; and if the absence of hair, mammæ, and two occipital condyles, in these other vertebrates were taken to imply a fundamental distinction, it might, in the absence of any known fossil links, be inferred that the Mammalia belonged to a separate phylum. But these differences are not held to negative the assumed relationship. Similarly among plants. We must not reject an hypothesis respecting a certain supposed type, because the existing types it must have been akin to present traits which it could not have had. We are justified in assuming, within limits, a hypothetical type, unlike existing types in traits of some importance. Hence results the answer to a criticism passed on the above argument, that it implies relations between the undeveloped and developed forms of the Jungermanniaceæ such as the facts do not show us. This objection is met on remembering that the types in which the supposed transition took place disappeared myriads of years ago.