DOCTRINE OF EVOLUTION AND RELATED IDEAS 489 



statements. The student must go to more extended texts 

 for the complete handling of this complex but most interesting 

 subject. 



Some important facts of individual development are; 



1. Each individual animal (with certain exceptions in 

 non-sexual reproduction), no matter how high in the scale 

 of life, starts its life as a single cell, similar in many respects 

 to the permanent single-celled protozoans, except that it has 

 the power of developing rapidly into its own peculiar species. 

 (See Pig. 250, I.) 



2. All higher forms agree likewise in the next step of their 

 development, which consists in the division of this single cell 

 into a simple mass of cells. This is known as the morula, which 

 is not unlike the adult, and highest stage, of such types as Eudor- 

 ina and Volvox (colonial protozoa. Pig. 250, 2). 



3. Practically all the higher animals pass next through 

 a stage in which the cells become differentiated into two layers, 

 more or less well defined and arranged in a kind of double sac . 

 — ectoderm on the outside and entoderm on the inside (see Pig. 

 13, A 4). This is known as the ga5<rwZo. Animals like the adult 

 Hydra (Pig. 81) are really a kind of permanent gastrula — some- 

 what modified in form to be sure, but a gastrula nevertheless. 



The facts thus far stated may be taken as suggesting the 

 following conclusions: 



1. All organisms, even the highest, begin life at essen- 

 tially the same point ; that is, as a single cell. This similarity of 

 individual origin indicates their fundamental kinship and simi- 

 larity of racial origin. 



2. The development of all the forms above the Protozoa 

 is parallel for at least a brief period; that is, through the morula 

 and the blastula stages (Pig. 13, 3 and 4). This parallelism of 

 development added to the similarity of origin points even more 

 strongly to their kinship. 



3. There is tendency for some forms to drop out of the race 

 and to become permanently fixed about certain of these stages: 

 as most protozoa at the single-celled stage, Volvox at the morula 

 stage, and sponges and hydra at the gastrula stage. Others go 

 on and introduce new steps of differentiation before reaching their 

 adult development (Pig. 250: 1-9). 



