September ii, 1890] 



NA TURE 



477 



donkey ; the Miocene Mesohippus about equalled a sheep ; 

 while Eohippus, from the Lower Eocene deposits, was no larger 

 than a fox. Not only is there good reason for holding that, as 

 a rule, larger iinimals are descended from ancestors of smaller 

 size, but there is also much evidence to show that increase in size 

 beyond certain limits is disadvantageous, and may lead to 

 destruction rather than to survival. It has happened more than 

 once in the history of the world, and in more than one group of 

 animals, that gigantic stature has been attained immediately 

 before extinction of the group — a final and tremendous effort to 

 secure survival, but a despairing and unsuccessful one. The 

 Ichthyosauri, Plesiosauri, and other extinct reptilian groups, the 

 Moas, and the huge extinct Edentates, are well-known examples, 

 to which before long will be added the elephants and the whales, 

 and, it may be, ironclads as well. 



The whole question of the influence of size is of the greatest 

 possible interest and importance, and it is greatly to be hoped 

 that it will not be permitted to remain in its present uncertain 

 and unsatisfactory condition. 



It may be suggested that Amphioxus is an animal which has 

 undergone reduction in size, and that its structural simplicity 

 may, like that of Limapontia, be due, in part at least, to this 

 reduction. Such evidence as we have tells against this suggestion ; 

 the first system to undergo degeneration in consequence of a 

 reduction in size is the respiratory, and the respiratory organs of 

 Amphioxus, though very simple, are also, for a vertebrate, un- 

 usually extensive. 



We have now considered the more important of the influences 

 which are recognized as affecting developmental history in such 

 a way as to render the recapitulation of ancestral stages less 

 complete than it might otherwise be, which tend to prevent 

 ontogeny from correctly repeating the phylogenetic history. It 

 may at this point reasonably be asked whether there is any way 

 of distinguishing the palingenetic history from the later ceno- 

 genetic modifications grafted on to it — any test by which we can 

 determine whether a given larval character is or is not ancestral. 



Most assuredly there is no one rule, no single test, that will 

 apply in all cases ; but there are certain considerations which 

 will help us, and which should be kept in view. 



A character that is of general occurrence among the members 

 of a group, both high and low, may reasonably be regarded as 

 having strong claims to ancestral rank ; claims that are greatly 

 strengthened if it occurs at corresponding developmental periods 

 in all cases ; and still more if it occurs equally in forms that 

 hatch early as free larvae, and in forms with large eggs, which 

 develop directly into the adult. As examples of such characters 

 may be cited the mode of formation and relations of the noto- 

 chord, and of the gill clefts of vertebrates, which satisfy all the 

 conditions mentioned. 



Characters that are transitory in certain groups, but retained 

 throughout life in allied groups, may, with tolerable certainty, 

 be regarded as ancestral for the former : for instance, the 

 symmetrical position of the eyes in young flat fish, the spiral 

 shell of the young limpet, the superficial positions of the madre- 

 porite in Elasipodous Holothurians, or the suckerless condition 

 of the ambulacral feet in many Echinoderms. 



A more important consideration is that if the developmental 

 changes are to be interpreted as a correct record of ancestral 

 history, then the several stages must be possible ones, the history 

 must be one that could actually have occurred, i.e. the several 

 steps of the history as reconstructed must form a series, all the 

 stages of which are practicable ones. 



Natural selection explains the actual structure of a complex 

 organ as having been acquired by the preservation of a series of 

 stages, each a distinct, if slight, advance on the stage imme- 

 diately preceding it — an advance so distinct as to confer on its 

 possessor an appreciable advantage in the struggle for existence. 

 It is not enough that the ultimate stage should be more advanta- 

 geous than the initial or earlier condition, but each intermediate 

 stage must also be a distinct advance. If, then, the development 

 of an organ is strictly recapitulatory, it should present to us a 

 series of stages, each of which is not merely functional, but a 

 distinct advance on the stage immediately preceding it. Inter- 

 mediate stages, e.g. the oesophagus of the tadpole, which are 

 not and could not be functional, can form no part of an ancestral 

 series— a consideration well expressed by Sedgwick ^ thus : 



' " On the Early Development of the Anterior Part of the Wolffian Duct 

 and Body in the Chick" {Quarterly Journal of Microscopical Science, 

 vol. xxi., 1881, p. 456). 



" Any phylogenetic hypothesis which presents difficulties from 

 a physiological standpoint must be regarded as very provisional 

 indeed." 



A good example of an embryological series fulfilling these 

 conditions is afforded by the development of the eye in the 

 higher Cephalopoda. The earliest stage consists in the depres- 

 sion of a slightly modified patch of skin ; round the edge of the 

 patch the epidermis becomes raised up as a rim ; this gradually 

 grows inwards from all sides, so that the depressed patch now 

 forms a pit, communicating with the exterior through a small 

 hole or mouth. By further growth the mouth of the pit becomes 

 still more narrowed, and ultimately completely closed, so that 

 the pit becomes converted into a closed sac or vesicle ; at the 

 point at which final closure occurs, formation of cuticle takes 

 place, which projects as a small transparent drop into the cavity 

 of the sac ; by formation of concentric layers of cuticle, this 

 drop becomes enlarged into the spherical transparent lens of the 

 eye, and the development is completed by histological changes 

 in the inner wall of the vesicle, which convert it into the retina, 

 and by the formation of folds of skin around the eye, which 

 become the iris and the eyelids respectively. 



Each stage in this developmental history is a distinct advance, 

 physiologically, on the preceding stage, and, furthermore, each 

 stage is retained at the present day as the permanent condition 

 of the eye in some member of the group MoUusca. 



The earliest stage, in which the eye is merely a slightly 

 depressed and slightly modified patch of skin, represents the 

 simplest condition of the MoUuscan eye, and is retained through- 

 out life in Solen. The stage in which the eye is a pit, with 

 widely open mouth, is retained in the limpet ; it is a distinct 

 advance on the former, as through the greater depression the 

 sensory cells are less exposed to accidental injury. 



The narrowing of the mouth of the pit in the next stage is a 

 simple change, but a very important step forwards. Up to this 

 point the eye has served to distinguish light from darkness, but 

 the formation of an image has been impossible. Now, owing 

 to the smallness of the aperture, and the pigmentation of the 

 walls of the pit which accompanies the change, light from 'any 

 one part of an object can only fall on one particular part of the 

 inner wall of the pit or retina, and so an image, though a dim 

 one, is formed. This type of eye is permanently retained in the 

 Nautilus. 



The closing of the mouth of the pit by a transparent mem- 

 brane will not affect the optical properties of the eye, and will 

 be a gain, as it will prevent the entrance of foreign bodies into 

 the cavity of the eye. 



The formation of the lens by deposit of cuticle is the next 

 step. The gain here is increased distinctness and increased 

 brightness of the image, for the lens will focus the rays of light 

 more sharply on the retina, and will allow a greater quantity of 

 light, a larger pencil of rays from each part of the object, to 

 reach the corresponding part of the retina. The eye is now in 

 the condition in which it remains throughout life in the snail 

 and other gastropods. Finally the formation of the folds of 

 skin known as iris and eyelids provides for the better protection 

 of the eye, and is a clear advance on the somewhat clumsy 

 method of withdrawal seen in the snail. 



The development of the vertebrate liver is another good but 

 simpler example. The most primitive form of the liver is that 

 of Amphioxus, in which it is present as a simple saccular diver- 

 ticulum of the intestinal canal, with its wall consisting of a 

 single layer of cells, and with blood-vessels on its outer surface. 

 The earliest stage in the formation of the liver in higher ver- 

 tebrates — the frog, for instance — is practically identical with 

 this. In the frog the next stage consists in folding of the wall 

 of the sac, which increases the efficiency of the organ by in- 

 creasing the extent of surface in contact with the blood-vessels. 

 The adult condition is attained simply by a continuance of this 

 process ; the foldings of the wall becoming more and more 

 complicated, but the essential structure remaining the same — a 

 single layer of epithelial cells in contact on one side with blood- 

 vessels, and bounding on the other directly or indirectly the 

 cavity of the alimentary canal. 



It is not always possible to point out the particular advantage 

 gained at each step even when a complete developmental series 

 is known to us, but in such cases, as, for instance, in Orbitolites, 

 our difficulties arise chiefly from ignorance of the particular con- 

 ditions that confer advantage in the struggle for existence in the 

 case of the forms we are dealing with. 



NO. T089, VOL. 42] 



