CiiAP. II. SUCCESSION OF CHARACTERS. 173 



Let ns see whether it is not possible to introduce more precision in this matter. 

 Taking for granted that Avhat I have said about the characteristics of the natural 

 groups in the animal kingdom is correct, that we have, 1st, four great typical 

 branches of the animal kingdom, characterized by different plans of structure ; 

 2d, classes, characterized by the ways in which and the means with which these 

 plans of structure are executed ; 3d, orders, characterized by the degrees of simplicity 

 or complication of that structure ; 4th, families, characterized by differences of form, 

 or by the structural peculiarities determining form ; 5th, genera, characterized ])y 

 ultimate peculiarities of structure in the parts of the body; Gth, species, charac- 

 terized by relations and proportions of parts among themselves, and of the indi- 

 viduals to one another and to the surrounding mediums; we reach, finally, the 

 individuals, which, for the time being, represent not only the species with all 

 their varieties, and variations of age, sex, size, etc., but also the characteristic features 

 of all the higher groups. We have thus, at one end of the series, the most com- 

 prehensive categories of the structure of animals, wliile at the other end we meet 

 individual beings. Individuality on one side, the most extensive divisions of the 

 animal kin^-dom on the other. Now, to beo-in our critical examination of the 

 progress of Hfe in its successive manifestations with the extremes, is it not plain, 

 from all we know of Embryology, that individualization is the first requirement 

 of all reproduction and multiplication, and that an individual germ, (or a number 

 of them,) an ovarian egg, or a bud, is first formed and becomes distinct as an 

 individual from the body of the parent, before it assumes either the charactere 

 of its great type or those of its class, order, etc. ? This fact is of great significance, 

 as showing the importance of individuality in nature. Next, it is true, we perceive 

 generally the outlines of the plan of structure, before it becomes apparent in 

 what manner that plan is to be carried out ; the character of the type is marked 

 out, in its most general features, before that of the class can be recognized with 

 any degree of precision. Upon this fact, we may base one of the most important 

 generalizations in Embrvologv. 



It has been maintained, in the most general terms, that the liighcr animals 

 pass durhig their development through all the phases characteristic of the inferior 

 classes. Put in this form, no statement can be further from the truth, and yet 

 there are decided relations within certain limits, between the embrAonic stages of 

 growth of higher animals and the permanent characters of others of an inferior 

 grade. Now the fact mentioned above, enables us to mark with precision the limits 

 within which these relations may be traced. As eggs, in their primitive condi- 

 tion, animals do not differ one from the other; but as soon as the embryo has 

 begun to show any characteristic features, it prei^ents such peculiarities as dis- 

 tiuguisli its type. It cannot, therefore, be said that any animal passes tlu-ough 



