in the Animal Kingdom. 135 



The cross lines (arcs) indicate, at the points where they cut 

 the curves, corresponding stages of development. It is in cor- 

 responding stages of development that resemblances occur. 



By the above diagram (Fig. 11.), resemblances between orga- 

 nized structures, admitting of comparison, are seen to be in the 

 inverse ratio of the age, the curves having a tendency to coin- 

 cide, in proportion as they approach the point A ; and the causes 

 of variety, are seen to be resolvable into differences in direction 

 and degree of development. 



There are many purposes to which it may be applied, such 

 as a comparison of the development of the primary divisions of 

 the animal or vegetable kingdom (Classes), or of any of their 

 subdivisions ; of corresponding layers of the Germinal Mem- 

 brane ; of systems of organs or single organs, in different ani- 

 mals, or in different plants ; or of systems of organs or single 

 organs, originating in the same Fundamental organ, of the same 

 animal or the same plant. 



Perhaps the chief use of this diagram consists, firstly, in as- 

 sisting the understanding, by affording something like an object, 

 to which descriptions may be referred ; and, therefore, second- 

 ly, in assisting the memory, by means of association. 



Let us, for example, refer to it a fact mentioned by Oken,* 

 and since by Burmeister,f that certain Insects, differing in the 

 degree of elaboration in the perfect state, differ in a corresponding 

 manner as larva also. Suppose that at the point A, four germs 

 of different Insects coincide essentially in fundamental form. 

 Let the curves represent their development in larval (i.e. a part of 

 embryonal) life; and let the germs be those respectively of one of 

 the Diptera, Hymenoptera, Coleoptera, and Lepidoptera. The 

 larvse of the Diptera (gnat, house-fly), whose development is re- 

 presented by the curve A B, resemble the footless and headless 

 Leech; those of the Hymenoptera (bee, wasp, ant) A C, may be 

 compared to Nais, in which there is a distinct head, but the feet 

 are wanting ; those of the Coleoptera (May and caddis-flies), 

 A D, are comparable to a third grade of the Annulata, " re- 

 siding in tubes, and furnished with large bundles of gills." 



Allgemeine Naturgeschichte fur alle Stande," 8vo. Bandiv. . 469-470. 

 t Loco citato, pp. 419, 420. 



