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THE POPULAR SCIENCE MONTHLY.— SUPPLEMENT. 



show that they are all more or less closely related 

 together by a chain of similarity of a very marked 

 and unmistakable character; that in their sim- 

 plest forms they are indeed, in so far as our pow- 

 ers of observation enable us to know them, iden- 

 tical ; that in the lower grades of animal and 

 vegetable life they are so similar as to pass by 

 insensible gradations into each other; and that 

 in the higher forms, while they diverge most 

 widely in some of their aspects in the bodies be- 

 longing to the two great kingdoms of organic 

 Nature, and in the larger groups distinguishable 

 within each of them, yet it is still possible, from 

 the fundamental similarity of the phenomena, to 

 trace in the transitional forms of all their varie- 

 ties one great general plan of organization. 



In its simplest and earliest form that plan 

 comprises a minute mass of the common nitro- 

 genous hydrocarbon compound to which the 

 name of protoplasm has been given, exhibiting 

 the vital properties of assimilation, reproduction, 

 and irritability ; the second stage in this plan is 

 the nucleated and inclosed condition of the pro- 

 toplasmic mass in the organized cell. We next 

 recognize the differentiation of two productive 

 elements, and their combination for the formation 

 of a more highly-endowed organizing element in 

 the embryonic germ-sphere or cell ; and the 

 fourth stage of advance in the complexity of the 

 organizing phenomena is in the multiplication of 

 the fertilized embryo-cell, and its conversion into 

 continuous organized strata, by further histologi- 

 cal changes in which the morphological founda- 

 tions of the future embryo or new being are laid. 



I need not now recur to the further series of 

 complications in the formative process by which 

 the bilaminar blastoderm is developed, and be- 

 comes trilaminar or quadrilaminar, but only 

 recall to your recollection that while these sev- 

 eral states of the primordial condition of the 

 incipient animal pass insensibly into each other, 

 there is a pervading similarity in the nature 

 of the histological changes by which they are 

 reached, and that in the production of the end- 

 less variations of form assumed by the organs 

 and systems of different animals in the course of 

 their development, the process of cell-production, 

 multiplication, and differentiation, remains iden- 

 tical. The more obvious morphological changes 

 are of so similar a character throughout the 

 whole, and so nearly allied in the different larger 

 groups, that we. are led to regard them as placed 

 in some very close and intimate relation to the 

 inherent properties of the organic substance 

 which is their seat, and the ever-present influence 



of the vital conditions in which alone these prop- 

 erties manifest themselves. 



The formative or organizing property, there- 

 fore, resides in the living substance of every or- 

 ganized cell and in each of its component mole- 

 cules, and is a necessary part of the physical and 

 chemical constitution of the organizing elements 

 in the conditions of life ; and it scarcely needs to 

 be said that these conditions may be as varied as 

 the countless numbers of the molecules which 

 compose the smallest particles of their sub- 

 stance. But, setting aside all speculation of a 

 merely pangenetic kind, it appears to me that no 

 one could have engaged in the study of embryo- 

 logical development for any time without be- 

 coming convinced that the phenomena which 

 have been ascertained as to the first origin and 

 formation of textures and organs in any indi- 

 vidual animal are of so uniform a character as to 

 indicate forcibly a law of connection and conti- 

 nuity between them ; nor will his study of the 

 phenomena of development in different animals 

 have gone far before he is equally strongly con- 

 vinced of the similarity of plan in the develop- 

 ment of the larger groups, and, to some extent, 

 of the whole. I consider it impossible, therefore, 

 for any one to be a faithful student of embry- 

 ology, in the present state of science, without at 

 the same time becoming an evolutionist. There 

 may still be many difficulties, some inconsisten- 

 cies, and much to learn, and there may remain 

 beyond much which we shall never know; but I 

 cannot conceive any doctrine professing to bring 

 the phenomena of embryonic development within 

 a general law which is not, like the theory of 

 Darwin, consistent with their fundamental iden- 

 tity, their endless variability, their subjugation to 

 varying external influences and conditions, and 

 with the possibility of the transmission of the 

 vital conditions and properties, with all their 

 variations, from individual to individual, and, in 

 the long lapse of ages, from race to race. 



I regard it, therefore, as no exaggerated rep- 

 resentation of the present state of our knowledge 

 to say that the ontogenetic development of the 

 individual in the higher animals repeats in its 

 more general character, and in many of its spe- 

 cific phenomena, the phylogenetic development of 

 the race. If we admit the progressive nature of 

 the changes of development, their similarity in 

 different groups, and their common characters in 

 all animals, nay, even in some respects in both 

 plants and animals, we can scarcely refuse to 

 recognize the possibility of continuous derivation 

 in the history of their origin ; and however far 



