THE CLASSIFICATION OF ANIMALS 



7.6). There are always differences of opinion among the experts, even when 

 the same observations are involved, and many aspects of cla-ssification must 

 remain matters of judgment until more explicit information is forthcoming 

 than any now available. Yet, there has come to be an increasing degree of 

 agreement in taxonomy as knowledge of animal life and characteristics has 

 progressed. 



Evolutionary Interpretation of Classification. We may now ex- 

 amine the evolutionary implications of the natural or genetic classification, 

 which zoologists have constructed on the basis of structural resemblance. 

 This classification is, as we have seen, an effort to construct a family tree 

 of animal life. If certain phyla are placed together as Eumetazoa, the group- 

 ing means that they are regarded as closely related in ancestry. It is 

 possible, therefore, to consider classification as a statement of evolutionary 

 probabilities. 



Referring to Figure 7.4 as though it were a family tree, we may state that 

 the first great step in evolutionary progress of animals was the divergence 

 between forms that continued in the ancestral single-celled condition and gave 

 rise to the Protozoa, and those that attained in some way the multicellular 

 state and gave rise to the Metazoa. Within the latter line, the next major 

 divergence was between animals that continued in a primitive state of dif- 

 ferentiation, the Mesozoa and Parazoa, and forms that acquired a gut cavity, 

 along with other complexities, and became the progenitors of all the eumeta- 

 zoan phyla. Within this more advanced group next occurred a divergence 

 into two stems: one, developing radial symmetry, the Radiata, and the other, 

 with bilateral symmetry, the Bilateria. Again, within the great stem Bilateria, 

 forms possessing a body cavity arose from ancestors which had lacked such a 

 cavity; and this space evidently originated in various ways and became 

 variously specialized. Finally, we have come in the course of evolution to 

 the existing phyla with their subdivisions, and to the species and individual 

 animals of today. 



Many of these animals, in their fullest, adult development, resemble forms 

 which may be interpreted as having served as stages in the evolution of higher 

 animals. This existence, in the life cycles of complex forms, of developmental 

 stages which resemble those of much simpler animals is considered significant 

 in evolutionary interpretation and is the basis of the so-called Recapitulotion 

 Theory. This states, in essence, that in its own developmental sequence each 

 individual organism exhibits transitory stages which represent stages in the 

 evolutionary history of the group to which it belongs. According to this 

 theory, we should be able, within certain limits, to determine the phylogenetic 

 derivation of a group of animals by studying the embryonic development of 

 modern representatives of the group. Interpreted in this light, the existence 

 of a unicellular stage, the zygote, in the life cycle of every sexually repro- 

 ducing animal may be regarded as reflecting the probability that all multi- 

 cellular animals have descended ultimately from unicellular forebears. The 

 fact that all members of the phyla grouped as Bilateria (Fig. 7.4) exhibit in 



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