44.S 



NA TURE 



[September 6, 1900 



anatomists that morphological definitions are regarded as depend- 

 ing essentially on the successive phases of the development of 

 the parts under consideration. 



The morphological characters exhibited by a plant or animal 

 tend to be hereditarily transmitted from parents to offspring, 

 and the species is perpetuated. In each species the evolution of 

 an individual, through the developmental changes in the egg, 

 follows the same lines in all the individuals of the same species, 

 which possess therefore in common the features called 

 specific characters. The transmission of these characters is due, 

 according to the theory of Weismann, to certain properties 

 possessed by the chromosome constituents of the segmentation 

 nucleus in the fertilised ovum, named by him the germ plasm, 

 which is continued from one generation to another, and impresses 

 its specific character on the egg and on the plant or animal 

 developed from it. 



As has already been stated, the special tissues which build up 

 the bodies of the more complex organisms are evolved out of 

 cells which are at first simple in form and appearance. During 

 the evolution of the individual, cells become modified or differen- 

 tiated in structure and function, and so long as the differentiation 

 follows certain prescribed lines the morphological characters of 

 the species are preserved. We can readily conceive that, as the 

 process of specialisation is going on, modifications or variations 

 in groups of cells and the tissues derived from them, notwith- 

 standing the influence of heredity, may in an individual diverge 

 so far from that which is characteristic of the species as to 

 assume the arrangements found in another species, or even in 

 another order. Anatomists had indeed long recognised that 

 variations from the customary arrangement of parts occasionally 

 appeared, and they described such deviations from the current 

 descriptions as irregularities. 



Darwinian l^heory. 



The signification of the variations which arise in plants and 

 animals had not been apprehended until a flood of light was 

 thrown on the entire subject by the genius of Charles Darwin, 

 who formulated the wide-reaching theory that variations could 

 be transmitted by heredity to younger generations. In this 

 manner he conceived new characters would arise, accumulate, 

 and be perpetuated, which would in the course of time assume 

 specific importance. New species might thus be evolved out of 

 organisms originally distinct from them, and their specific 

 characters would in turn be transmitted to their descendants. 

 By a continuance of this process new species would multiply in 

 many directions, until at length from one or more originally sim- 

 ple forms the earth would become peopled by the infinite varieties 

 of plant and animal organisms which have in past ages inhabited, 

 or do at present inhabit, our globe. The Darwinian theory may 

 therefore be defined as Heredity modified and influenced by 

 Varialjility. It assumes that there is an heredity quality in the 

 egg, which, if we take the common fowl for an example, shall con- 

 tinue to produce similar fowls. Under conditions, of which we 

 are ignorant, which occasion molecule r changes in the cells and 

 tissues of the developing egg, variations might arise in the first 

 instance pr >bably slight, but becoming intensified in successive 

 generations, until at length the descendants would have lost the 

 characters of the fowl and have become another species. No 

 precise estimate has been arrived at. and indeed one does not 

 see how it is possible to obtain it, of the length of years which 

 might be required to convert a variation, capable of being trans- 

 mitted, into a new and definite specific character. 



The circumstances which, according to the Darwinian theory, 

 determined the perpetuation by hereditary transmission of a 

 variety and its assumption of a specific character depended, it 

 was argued, on whether it possessed such properties as enabled 

 the plant or animal in which it appeared to adapt itself more 

 readily to its environment, i.e. to the surrounding conditions. 

 If it were to be of use the organism in so far became 

 better adapted to hold its own in the struggle for 

 existence with its fellows and with the forces of nature 

 operating on it. Through the accumulation of useful 

 characters the specific variety was perpetuated by natural 

 selection, so long as the conditions were favourable for its exist- 

 ence, and it survived as being the best fitted to live. In the study 

 of the transmission of variations which may arise in the course 

 of development it should not be too exclusively thought that 

 only those variations are likely to be preserved which can be "of 

 service during the life of the individual, or in the perpetuation 

 of the species, and possibly available for the evolution of new 



NO. 1 6 10, VOL. 62] 



species. It should also be kept in mind that morphologicaf 

 characters can be transmitted by hereditary descent, which, 

 though doubtless of service in some bygone ancestor, are ir> 

 the new conditions of life of the species of no physio- 

 logical value. Our knowledge of the structural and func- 

 tional modifications to be found in the human body, in 

 connection with abnormalities and with tendencies or 

 predisposition to diseases of various kinds, teaches us that 

 characters which are of no use, and indeed detrimental to the 

 individual, may be hereditarily transmitted from parents to off- 

 spring through a succession of generations. 



Since the conception of the possibility of the evolution of new 

 species from pre-existing forms took possession of the minds of 

 naturalists, attempts have been made to trace out the lines on. 

 which it has proceeded. The first to give a systematic account of 

 what he conceived to be the order of succession in the evolutior> 

 of animals was Ernst Haeckel, of Jena, in a well-known treatise. 

 Memoirs on special departments of the subject, too numerous to 

 particularise, have subsequently appeared. The problem has been, 

 attacked along two different lines : the one by embryologists, 

 of whom may be named Kowalewsky, (iegenbaur, Dohrn, Ray 

 Lankester, Balfour and Gaskell, who with many others have 

 conducted careful and methodical inquiries into the stages of 

 development of numerous forms belonging to the two great 

 divisions of the animal kingdom. Invertebrates, as well as 

 vertebrates, have been carefully compared with each other ii> 

 the bearing of their development and structure on their affinities 

 and descent, and the possible sequence in the evolution of the 

 Vertebrata from the Invertebrata has been discussed. The 

 other method pursued by palaeontologists, of whom Huxley, 

 Marsh, Cope, Osborne and Traquair are prominent authorities, 

 has been the study of the extinct forms preserved in the rocks and 

 the comparison of their structure with each other and with that 

 of existing organisms. In the attempts to trace the line of 

 descent the imagination has not unfrequently been called into- 

 play in constructing various conflicting hypotheses. Though 

 from the nature of things the order of descent is, and without 

 doubt will continue to be, ever a matter of speculation and not 

 of demonstration, the study of the subject has been a valuable 

 intellectual exercise and a powerful stimulant to research. 



We know not as regards time when the fiat went forth, '* Lei 

 there be Life, and there was Life." All we can say is that it 

 must have been in the far-distant past, at a period so remote 

 from the present that the mind fails to grasp the duration of the 

 interval. Prior to its genesis our earth consisted of barren rock 

 and desolate ocean. When matter became endowed with Life, 

 with the capacity of self-maintenance and of resisting external 

 disintegrating forces, the face of nature began to undergo a 

 momentous change. Living organisms multiplied, the lancf 

 became covered with vegetation, and multitudinous varieties of 

 plants, from the humble fungus and moss to the stately palm and 

 oak, beautified its surface and fitted it to sustain higher kinds of 

 living beings. Animal forms appeared, in the first instance simple 

 in structure, to be followed by others more complex, until the 

 mammalian type was produced. The ocean also became peopled 

 with plant and animal organisms, from the microscopic diatom 

 to the huge leviathan. Plants and animals acted and reacted 

 on each other, on the atmosphere which surrounded them and orv 

 the earth on which they dwelt, the surface of which became 

 modified in character and aspect. At last Man came into 

 existence. His nerve-energy, in addition to regulating the pro- 

 cesses in his economy which he possesses in common with ani- 

 mals, was endowed with higher powers. When translated 

 into psychical activity it has enabled him throughout the ages 

 to progress from the condition of a rude savage to an ad- 

 vanced stage of civilisation ; to produce works in literature, art, 

 and the moral sciences which have exerted, and must continue 

 to exert, a lasting influence on the development of his higher 

 Being ; to make discoveries in physical science ; to acquire a 

 knowledge of the structure of the earth, of the ocean in its 

 changing aspects, of the atmosphere and the stellar universe, 

 of the chemical composition and physical properties of matter in 

 its various forms, and to analyse, comprehend, and subdue the 

 forces of nature. 



By the application of these discoveries to his own purposes 

 Man has, to a large extent, overcome time and space ; he has- 

 studded the ocean with steamships, girdled the earth with 

 electric wire, tunnelled the lofty Alps, spanned the Forth with 

 a bridge of steel, invented machines and founded industries of 



