48 evolution: the ages and tomorrow 



Here the criterions of progress can be based on the control 

 over the environment, the degree of independence of the 

 organism, and, above all, the increase of efficiency and total 

 capacities of the nervous system. All else is incidental. The 

 problem of multicellular organization was solved much 

 more quickly in animals than in plants. In fact, by the open- 

 ing of the Cambrian period (about one billion years ago), 

 while plants were still below the true multicellular level, 

 animals had already evolved the main primitive representa- 

 tives of animals without a backbone, the invertebrates. 



The first primitive vertebrate, the group to which man 

 belongs, appeared at about the time that plants had evolved 

 a moss-like stage. Living today are many intermediate forms 

 in the long phyletic (major kind) series from the amoeba to 

 man, and it is impossible to survey this series without being 

 impressed with the progress that is demonstrated. The fos- 

 sil record fills in here and there to make the series even more 

 impressive. Some primitive types that may still be similar to 

 the first multicellular forms are with us today: the semi- 

 multicellular sponges, and at yet more primitive stages those 

 relics, the Volvox colonies or the lowly blastula (hollow 

 ball) of our fresh water ponds. The jelly fishes, the corals, 

 and hydra, with their bodies organized in two tissue layers 

 folded inward to form a double-walled cup (gastrula), are 

 very active and numerous today; and yet they must be 

 quite close to the originator of the well-established multi- 

 cellular plan. And, more remarkable still, their plan is re- 

 peated and well set up in the embryos of animals all the way 

 up to and including man. 



Here in these simple animal forms is the beginning of a 

 wonderful series of nature's "inventions" which are to ap- 

 pear either in the embryonic or in the adult life of all higher 

 animal forms. Nature's "gadgets" are passed along. The flat- 

 worms take over the double-walled cup with its outer pro- 

 tective-sensory tissue and its inner digestive layer, utilize a 

 third tissue in between, flatten it all out, and change the 



