TRANSITION LINKS IN RELATION TO TYPES 401 



disuse. It merely bespeaks repetition, type, general plan, and the persistence which general plan necessitates. The 

 appearance and partial or complete disappearance by absorption or otherwise of developing foetal structures afford 

 additional proof in the same direction. 



§ 81. Transition Links in Relation to Types. 



The lancelet (Amphioxus lanceolatus) (Plate xciv.. Fig. 1, page 404) affords an example of a low type which 

 foreshadows all the higher vertebrates. This anomalous rudimentary creature is fish-Uke in its general appearance, 

 possessed of fins, has a rudimentary nervous system, a tiny brain, a notochord or incipient spinal colunm, blood- 

 vessels but no heart, an alimentary canal, and a well-defined, symmetrical muscular system. Notwithstanding all 

 this it is a degraded type, and displays little intelligence and httle activity. The following is the account given of 

 it and other low fish forms by Dr. Giinther : " The lancelet seems to be almost cosmopolitan within the temperate 

 and tropical zones. It inhabits shallow, sandy parts of the coast, and rarely exceeds three inches in length. It 

 belongs to the lowest sub-class of fishes, comprises one form only, and possesses a skeleton of the most primitive 

 type. The vertebral column is represented by a simple chorda dor sails or notochord only, which extends from one 

 extremity of the fish to the other, and, so far from being expanded into a cranial cavity, it is pointed at its anterior 

 end as well as at its posterior. There is no trace of vertebral segments or ribs." ^ 



Dr. Giinther describes other transitionary types, which may be mentioned in this connection from their 

 obvious bearing on development. " The skeleton of the Cyclostomata (lampreys and sea-hags) shows a consider- 

 able advance of development. It consists of a notochord, the anterior pointed end of which is wedged into the 

 base of a cranial capsule, partly membranous, partly cartilaginous. This skull, therefore, is not movable upon 

 the spinal column. No vertebral segmentation can be observed in the notochord, but neural arches are represented 

 by a series of cartilages on each side of the spinal cord. . . . The Chondropterygians exhibit a most extraordinary 

 diversity in the development of their vertebral column ; almost every degree of ossification, from a notochord 

 without a trace of annular structure to a series of completely ossified vertebrae, being found in this order. Sharks, 

 in which the notochord is persistent, are the Holocephali (if they be reckoned to this order, and the genera Notidanus 

 and Echinorhinus). Among the first, Chimiera monstrosa begins to show traces of segmentation ; but they are 

 limited to the outer sheath of the notochord, in which slender sub-ossified rings appear. In Notidanus, membranous 

 septa with a central vacuity cross the substance of the gelatinous notochord. In the other sharks the segmentation 

 is complete, each vertebra having a deep conical excavation in front and behind, with a central canal through 

 which the notochord is continued ; but the degree in which the primitive cartilage is replaced by concentric or 

 radiating lamellae of bone varies greatly in the various genera, and according to the age of the individuals. In 

 the rays all the vertebrae are completely ossified, and the anterior ones confluent into one continuous mass." ^ 



The rudimentary types referred to above form the natural connecting links in the great vertebrate series, and 

 as such are entitled to great consideration. They, moreover, illustrate successive stages in the development of 

 mammals in ntero, which is the subject at present imder discussion. In the early human embryo the vertebral 

 column is formed roimd a simple rod -like structure, the primitive skeletal axis or notochord, which in the majority 

 of vertebrate animals disappears more or less completely during development. 



The differentiations in the young or growing frog provide some of the best-known examples of transitional or 

 growing structures (Plate xciii.. Figs. 2 and 3, page 403). These are at once striking, interesting, and instructive. 

 The changes which occur in the development of the frog are spontaneous in their nature, and, in no sense, due to 

 irritation, extraneous stimulation, or environment. They are means to ends ; the object being to convert a fish-like, 

 water-breathing animal into a land animal with four limbs, which breathes air. The exigencies of the land 

 and water respectively demand the striking structural differences which characterise the tadpole and the frog. 

 But (and this is the important point) the differences are inaugurated and carried out in anticipation of the change 

 from the water to the land : they are not caused by the water and land, the so-called environments. What holds 

 true of the frog holds true of all animals constructed to walk, run, or leap on the land, to swim and dive in the 

 water, and to fly in the air. In all these cases, the organs of locomotion are specially formed in advance, and in 

 anticipation of the special function to be performed. There is no getting away from these facts. The obvious 

 conclusion is, that special external conditions are duly provided for according to a pre-arranged plan, nothing being 



1 Dr. Mastennan gives a somewhat similar description : " Tlie lancelet is a very small marine organism from one inch to an inch and a half in 

 leno-th. It is of an elongated, fish-like shape, ilattened laterally, the body being piano-symmetric. It is of a milky-white, semi-transparent 

 appearance, and a number of the organs may be seen through the skin in the hving animal. There are no definite external divisions of the body 

 The muscular system is well developed, there is no heart, but the bases of the afferent branchial vessels are contractile. The nervous system lie 

 immediately dorsal to the notochord, and consists of a long tube, the front portion of which forms a small hrain and the rest the spinal cord." 



2 " An Introduction to tlie Studjf of Fishes," by Albert C. L. (i, Gunthev. M.A., M.D., F.R.S., 1880. 



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