If we arrange the results of palæontological inquiry which have now been stated in the form of a table such as that which is given on the following page, the significance of the succession of astacomorphous forms, in time, becomes apparent. {345}
SUCCESSIVE FORMS OF THE ASTACOMORPHOUS TYPE.
If an Astacomorphous crustacean, having characters intermediate between those of Eryma and those of Pseudastacus, existed in the Triassic epoch or earlier; if it gradually diverged into Pseudastacine and Erymoid forms; if these again took on Astacine and Homarine {346} characters, and finally ended in the existing Potamobiidæ and Homarina, the fossil forms left in the track of this process of evolution would be very much what they actually are. Up to the end of the Mesozoic epoch the only known Potamobiidæ are marine animals. And we have already seen that the facts of distribution suggest the hypothesis that they must have been so, at least up to this time.
Thus, with respect to the Ætiology of the crayfishes, all the known facts are in harmony with the requirements of the hypothesis that they have been gradually evolved in the course of the Mesozoic and subsequent epochs of the world’s history from a primitive Astacomorphous form.
And it is well to reflect that the only alternative supposition is, that these numerous successive and coexistent forms of insignificant animals, the differences of which require careful study for their discrimination, have been separately and independently fabricated, and put into the localities in which we find them. By whatever verbal fog the question at issue may be hidden, this is the real nature of the dilemma presented to us not only by the crayfish, but by every animal and by every plant; from man to the humblest animalcule; from the spreading beech and towering pine to the Micrococci which lie at the limit of microscopic visibility.
NOTES.
NOTE I., CHAPTER I., p. [17]. THE CHEMICAL COMPOSITION OF THE EXOSKELETON.
The harder parts of the exoskeleton of the crayfish contain rather more than half their weight of calcareous salts. Of these nearly seven-eighths consist of carbonate of lime, the rest being phosphate of lime.
The animal matter consists for the most part of a peculiar substance termed Chitin, which enters into the composition of the hard parts not only of the Arthropoda in general but of many other invertebrated animals. Chitin is not dissolved even by hot caustic alkalies, whence the use of solutions of caustic potash and soda in cleaning the skeletons of crayfishes. It is soluble in cold concentrated hydrochloric acid without change, and may be precipitated from its solution by the addition of water.