ANIMAL EVOLUTION 



533 



are not inherent in the animals themselves, 

 but are a direct response to their physical 

 surroundings. 



If we admit the fundamental unity of 

 animal life, we also must suppose that 

 every animal type bears a perfectly definite 

 relationship to every other, not merely 

 to one or two which are more or less 

 similar in structure. 



The developmental history of animals 

 commonly is shown in the form of a 

 branching tree-like figure, the branches 

 representing types which are assumed to 

 be anomalous and of dubious relation- 

 ships. Such a delineation denies the 

 essential unity of animals taken as a 

 whole. It assumes an aimless and hap- 

 hazard plan in animal development wholly 

 contrary to what we find in the domain of 

 physics and of chemistry which together 

 form the basis of all organic life. A 

 logical view of the interrelationships of 

 animals must be one that does not run 

 counter to what is evidenced elsewhere. 



But is there any evidence to show that 

 the development of animals is reducible 

 to any orderly plan? Can the various 

 animal types be so arranged as to show a 

 logical relationship each with all the 

 rest? 



THE DEVELOPMENT OF BILATERAL FROM 

 RADIAL FORMS 



In tracing animal development we 

 considered the single celled animals, or 

 protozoans, the sponges, and the radially 

 symmetrical animals. The occurrence of 

 a gastrula stage in all bilateral animals 

 was assumed to show their origin from 

 the radial type. How could the develop- 

 ment of the numerous types of bilateral 

 animals from a radial progenitor proceed? 



If colonial ccelenterate animals should 

 develop a persistent and considerable 

 defect in the ontogeny whereby the units 

 lost their perfect radial symmetry and 



acquired a pronounced bilateral symmetry, 

 this would give rise to animals of four 

 main types which, though chiefly bi- 

 lateral, would retain to a considerable 

 extent evidences of the fundamental radial 

 symmetry. 



These four new animal types, inter- 

 mediate between radially and bilaterally 

 symmetrical animals, but nearer the latter, 

 would take the form of: 



(i) Bilateral animals constituting a 

 linear, more or less unified, colony. 



(2.) Bilateral animals in which the 

 colony formation is inverted, the budding 

 off of the new units taking place within 

 the original unit. 



(3) Solitary bilateral animals each rep- 

 resenting a dissociated ccelenterate unit. 



(4) Bilateral animals with the colonial 

 habit, though independent of each other. 



These four types are all foreshadowed 

 in forms occurring among the ccelenterates 

 themselves, and furthermore in ccelen- 

 terates in which the radial structure is 

 slightly modified, being not quite perfect, 

 but symmetrical on either side of a plane 

 passing longitudinally through the units; 

 in other words, in radially symmetrical 

 animals which have acquired a partial 

 bilateral symmetry. 



What could be more natural than that 

 radially symmetrical animals with a 

 partial bilateral symmetry would pass 

 over into bilateral animals with a partial 

 radial symmetry in all the different forms 

 in which the existence of a creature with 

 such a mixed symmetry is possible? 



Among the animals of the present day 

 all four of these chief intermediate types 

 are represented. 



(1) The tapeworms or segmented ces- 

 todes form a linear colony having con- 

 tinuous growth which is so very similar 

 to the partially unified chain of young in 

 the common jelly-fish (the so-called 

 strobila of Aurellia) as to leave little 



