THE EARLIEST VERTEBRATED ANIMALS. 101

more? How can we account for the absence, not only of transitional forms in great numbers between them, but also of all modified descendants from those transitional forms? Admitting that the ancestral line uniting a reptile and a mammal may have been lost, how is it that the reptilian ancestor and the mammalian ancestor only have left descendants, so that a wide gap exists between mammals and reptiles? This difficulty appears formidable at first sight; but it vanishes if we consider the probabilities of the case.

All the more simple forms of life are aquatic or oceanic; the world of waters teems everywhere with lowly differentiated and lowly specialized organisms.

We may picture to ourselves the earliest representative of the Vertebrata as a fish-like creature, such as the Amphioxus or Lancelet, without limbs, but with a segmented cartilage-like rod representing its backbone, with ciliated respiratory clefts resembling the pharynx of an ascidian, and with numerous delicate tentacles around the margin of the mouthfish-like in its outward form (a circumstance clearly correlated with its mode of life), but otherwise almost as unlike any division of specialized fish as it is unlike an amphibian. Such a creature may have existed in countless millions, just as the Pteropods of the Arctic seas now exist, except that it could, in

102

THE EFFECT OF COMPETITION.

the early period of the history of evolution, which we are endeavouring to realize, have had no great whale-like enemies to keep down its numbers as those of the existing Pteropods are kept down. At such a period it is even improbable that any invertebrate would have been much, if any, more highly specialized, since there can be no reason why one mode of specialization should have been in advance

of

any other. If time had then sufficed for the formation of a cuttlefish, there is no reason why it should not have sufficed for the development of a highly organized vertebrate fish.

Amongst these millions of lancelet-like creatures some would vary; some may have lost their incipient segmentation and may have become invertebrates again; others may have perfected it, and have become evolved, by the action of the laws that have hitherto been discussed, into vertebrate forms. All these would exhibit a notochord inherited from their ancestors; and all would have preserved more or less of their elliptical form, so advantageous to them in aiding their progress through the water. Before any very great divergence had occurred from the lancelet type, some of the forms, hitherto unchecked by competition, would come into competition with each other, and those which possessed any considerable advantages over others would eventually occupy their place by their extermina

DISPERSION AN AID TO EVOLUTION. 103

tion. Several forms would undoubtedly eventually survive; but many would not succeed in the struggle

for life under any one set of conditions, and no great change of conditions would occur until the new-formed vertebrates became sufficiently specialized to migrate into new localities.

The first form of vertebrated animal may be supposed to have become widely distributed, as its gradual migration, or its independent origin under similar conditions from similar ancestors, would as yet be unchecked by enemies. Similar variations would undoubtedly occur in distant parts of the area stocked by our primitive form; and the same conditions which favoured one form in one region would favour a similar form in another.

The importance of a wide area and facility for dispersion in aiding evolution is probably very great this is exemplified by one of the most remarkable floras in the world, that of New Zealand. The plants of this island are in marked contrast with those of Australia: more than a third are unisexual; and nearly all have inconspicuous, slightly differentiated flowers*. Although many are highly variable, little or no progress seems to have occurred towards improvement, and the more remarkable types are gradually becoming exterminated by the invasion of European weeds. It appears as if * J. D. Hooker, Flora of New Zealand,' p. xxviii.

6

104

THE LAW OF PRIORITY.

isolation alone had enabled them to maintain their

ground.

As the power of migration increased and competition came again into play, the same process of extermination would weed out all the less specialized forms which were not protected by isolation or by some special conditions favourable to their peculiar natures. As each place in the polity of nature became stocked with already specialized forms, new forms would have no power to invade it, unless equally strong, equally adapted, and equally specialized. As some two or three forms descended from the Amphioxus-like ancestor would have the start of all others, and would become widely spread because no enemies existed, their descendants would always prevent any subsequent process of specialization amongst lower forms. The descendants of the more highly developed organisms would gradually become modified to occupy every space. It is the principle of priority: a stronger race, a more developed race, have already occupied every portion of an area; and hence no new forms rise to compete successfully from amongst lower or less-specialized creatures. All great improvement is checked in the less perfect forms by their soon coming into contact with the more perfect. It is the slowness and gradual nature of evolution which prevents the passage of lowly specialized types through any long

LOWLY ORGANIZED FORMS PERSIST. 105

series of changes, as such forms would always soon come into competition with species already more highly gifted to succeed than they could be. As soon as a vacancy occurs in the polity of nature for a new form, it becomes occupied by creatures of a type most nearly adapted for it, and no opportunity is left for the improvement of a lowly developed one. It will be perfectly clear to all that the apes of Western Africa would be quite unable to become improved in the direction of tameness and the development of a milder disposition and a less arboreal life as long as man occupies the country; so we have no reason to believe that the Amphioxus could leave its home burrowed in the sand, and develop fins for swimming in the water, as long as hundreds of highly specialized enemies exist.

It is easily seen that rudimentary fins, by enabling the primitive lancelet form to leave the bed of the sea, would have been advantageous whilst no fish provided with more perfect fins existed; on the other hand, such modifications occurring now, would only lead to the destruction of the fish with imperfect fins by those in which these organs have become perfected, the only safety for the little lancelet being its habit of remaining buried in the sand.

The existence of vast numbers of lowly organized