CONSTITUTION OF THE ANIMAL AND PLANT KINGDOMS. 55 



with other divisions or plans of the kingdom. The molluscoids or 

 " sea-squirts " themselves, for example, appear to be related, through 

 a curious worm-like creature, named Balanoglossus, to the worms on 

 the one hand, and to the star-fish group (Echinodermata) on the 

 other. Or, if we select the last-named group itself, we may discover 

 that the star-fishes and sea-urchins are not more isolated from other 

 types than are the vertebrates. The star-fishes, in fact, present 

 many points of affinity to certain worm-like forms ; and their 

 development, to be hereafter alluded to, clearly relates them, in the 

 eyes of the naturalists, to lower types of animal life. Again, the 

 lowest animals, or Protozoa, appear to be linked to the Ccelenterate 

 type (or that of the zoophytes, corals, sea-anemones, &c.) through 

 the sponges, which unite, in a most characteristic fashion, the fea- 

 tures of the lowest forms with organisms of a higher grade. And, 

 lastly, as amongst the worms we find the roots of the star-fish type, 

 so in that class also we discover the beginnings of the great 

 Articulate plan, which possesses the insects, crustaceans, and allied 

 animals as its chief representatives. As has well been remarked, 

 " it may reasonably be doubted whether any form of animal life 

 remains to be discovered which will not be found to accord with 

 one or other of the common plans now known. But, at the same 

 time, this increase of knowledge has abolished the broad lines of 

 demarcation which formerly appeared to separate one common plan 

 from another." 



Lastly, it will be shown in future chapters that the various 

 animal types start in their development from a common basis, and 

 agree in the earlier and essential stages of their progress towards 

 their adult forms. There is a literally amazing likeness to be dis- 

 cerned between the early stages of the development of many animals 

 which, as adults, and as belonging to different types, present not 

 the slightest resemblances to one another. Each animal, in fact, 

 traced backwards in its history, " approaches the earlier stages of all 

 the rest ; " that is to say, " all start from a common morphological 

 type, and, even in their extremest divergence, retain traces of their 

 primitive unity." Such unity will form the special subject of the 

 succeeding chapter, when the common and universal matter of life, or 

 protoplasm, is discussed in detail. 



It may thus be demonstrated as a fact, and as a matter removed 

 entirely from the domain of theory and hypothesis, that, whilst the 

 great world of animal life exhibits a constitution, in the study of which 

 its component elements are seen to be resolvable into several distinct 

 " types " or " plans " of structure, the development of these types has 

 followed a pathway and progress comparable to the growth of a tree. 

 The connections between types and the existence of intermediate and 

 transitional groups of animals, apparently belonging to one type when 



