CHAPTER IX 

 THE EVOLUTION OF THE VERTEBRATE EYE 



SIR EDWIN RAY LANKESTER (1847-1929) (Fig. 240), One of the foremost 

 British naturalists of the last generation, made outstanding contributions 

 to the subject-matter of this chapter. The origin of the vertebrate eye has long 

 been a puzzle and indeed still is ; and Lankester was one of the first to introduce 

 rationalism into the problem which had been largely speculative up to his tiine. 

 He sviggested that in the early Proto-chordates, transparent marine animals, 

 an eye associated with the central nervovis system would be a more plastic 

 organ than one derived from the integumentary epithelium and as effective 

 optically in organisms of this type ; as the bodies of Vertebrates become opaque, 

 migration of the eye towards the svirface became an obvious evolutionary 

 expedient. He was an example of that erudite type of scientist who was yet able 

 to popularize his philosophy, a type in which Britain has always been rich. His 

 academic career was full — Professor of Zoology and Comparative Anatomy at 

 University College London (1874-90), Linacre Professor of Comparative Anatomy 

 at Oxford (1891-98), director of the Xatural History Department of the British 

 Museum (1898-1907), and much of that time FuUerian Professor of Physiology 

 and Comparative Anatomy at the Royal Institution, London. He founded the 

 Marine Biological Association in 1884, was its President in 1892, and received 

 the Royal (1885) and Copley (1913) Medals of the Royal Society. 



The VERTEBRATE PHYLUM is of cnormous anticjuity and stems from the 

 primitive Agnatha, jaw-less pre-fishes, the fossil remnants of which are 400 

 million years old and are found abundantly in ancient Silurian rocks. Their 

 ancestors are unknowTi ^ but their descendants have become the lords of the 

 earth. It is interesting that as a general rule evolution proceeds through primi- 

 tive forms which, because of their simplicity and plasticity, have the jDOtentiality 

 to evolve into more highly differentiated forms ; but these latter, because of 

 their high differentiation and consequent superior equipment, can exterminate 

 their primitive forebears in the struggle for existence, but for the same reason 

 are incapable of further differentiation. The tendency is therefore for evolution 

 to proceed from primitive forms which have become largely extinct, producing 

 in its progress a series of evolutionary dead-ends each showing different highly 

 developed tj-pes of adaptive mechanisms designed to meet different specialized 

 circumstances. 



During recent years the views of zoologists on evolution within the vertebrate 

 phylum have changed considerably and it is probable that they have not yet 

 finally crystallized (Romer, 1947 ; Trewavas et al., 1955) (Fig. 241). It would 

 seem established, however, that the most archaic vertebrates are the worm-like 

 Agnatha, pre-fishes without jaws or limbs, which survive to-day in the primitive 



^ At one time or another the ancestry of Vertebrates has been sought in almost 

 every invertebrate group, particularly annelid worms, Arthropods (especially Arachnids 

 through Eurypterids). Perhaps the most reasonable theory, however, ascribes a com- 

 mon origin to the larvaj of the simplest Chordates and those of Echinoderms, despite the 

 vast and obvious discrepancy between the adults in each phylum. Palaeontology, 

 however, provides no record of such tiny, soft -bodied creatures as these larvae since they 

 are incapable of preservation as fossils (see Romer, 1947). 



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