LEADING GROUPS OF EXISTING SYSTEMS 183 



plans of structure are executed; 3d, orders, characterized by the de- 

 grees of simplicity or complication of that structure; 4th, families, 

 characterized by differences of form, or by the structural peculiarities 

 determining form; 5th, genera, characterized by ultimate peculiari- 

 ties of structure in the parts of the body; 6th, species, characterized 

 by relations and proportions of parts among themselves, and of the 

 individuals 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 comprehensive cate- 

 gories of the structure of animals, while at the other end we meet 

 individual beings. Individuality on one side, the most extensive divi- 

 sions of the animal kingdom on the other. Now, to begin our critical 

 examination of the progress of life 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 

 characters 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 Embryology. 



It has been maintained in the most general terms that the higher 

 animals pass during their development through all the phases charac- 

 teristic 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 embryonic stages of growth of higher ani- 

 mals 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 condition, animals do not differ one from the other; but 

 as soon as the embryo has begun to show any characteristic features 



