THE EVOLUTION OF ANIMALS. 387 



% catch the sound waves. In man these muscles are so 

 reduced as to be of no value. Most animals have many 

 rudimentary remnants of organs which are useful in other 

 apparently related animals. It is said that man alone 

 has several hundred such rudimentary structures. The 

 rudimentary eyes of fishes and Crustacea in caves, and 

 the almost or entirely reduced organs of many parasites 

 are mute testimony of the loss of organs once useful, 

 through changed life conditions. In other words, vestigial 

 organs are also evidence of evolution of animals into 

 adjustment with the surroundings. 



385. Evidences from Embryology. The study of em- 

 bryology the course of life in the individual has 

 probably furnished us the most suggestive evidence of the 

 evolution of animals. The main facts, and the use that 

 has been made of them in reaching our conclusions, may 

 be expressed in a few brief 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 Fig. 160, /.) 



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 Eudorina and Volvox (colonial 

 protozoa, Fig. 160, 2). 



