504 POPULAR SCIENCE MONTHLY. 



must have been reached, as a stage superior to the minute single-celled 

 animal, or the immobile plant, fed with inorganic nutriment. 



If we may then accept it as inevitable that organic evolution every- 

 where, if sufficiently advanced, must have reached the stage of the 

 metazoon animal, this may be taken as the necessary basis of higher 

 progress in any life-bearing planet. In the metazoon we have a crea- 

 ture consuming organic food, which it is necessary to seek, and thus 

 needing powers of self-motion, either of the body as a whole or of its 

 members. And in any planet in which beings equivalent to man ap- 

 peared the faculty of consciousness must have been equally necessary 

 at an early stage, as a highly advantageous aid in the struggle for 

 existence. 



This type of life once attained, it formed a fertile field for the 

 operation of the principle of natural selection. Upon the earth, and 

 presumably everywhere, it developed into innumerable forms, each 

 adapted to some passing or permanent condition of the environment. 

 Assuming that the agencies of internal organic activity were every- 

 where much the same — including active chemical change, due to oxi- 

 dation or something similar, vascular organs for the conveyance of 

 nutriment to the wasting tissues, apparatus for sensation and motion, 

 and the like — and that these led to the development of specialized 

 organs equivalent to the lungs, the heart, the brain, etc., we shall con- 

 fine ourselves here to the subject of variation in outward form and 

 condition. 



Even in this there are a multitude of relations to consider, and we 

 can deal here only with those of general character. A main one is that 

 of activity as contrasted with inactivity. Many of the new forms 

 became sessile animals, their only active parts being tentacles or other 

 organs of offense and defense. Others became free-moving animals. 

 Of the two types the latter was evidently the best adapted to high 

 development, both physical and mental, its free motion greatly diver- 

 sifying its environment and bringing it into much more varied rela- 

 tions than could be enjoyed by the plant-like sessile forms. The more 

 active the animal, the more diversified its powers of motion, the more 

 acute and varied its organs of sense, the more alert its powers of con- 

 sciousness, the higher seemingly would be its position in the ranks of 

 life and the superior its opportunities for upward progress. And this 

 rule must have jorevailed not only on the earth, but throughout the 

 universe. 



This being the case, not alone the sessile, but the sluggish, forms 

 were at a disadvantage as compared with the active. Anything, then, 

 likely to prevent rapidity and diversity of motion must have acted as 

 a check to progress. Activity is essential to the most effective offen- 

 sive powers, and upon these the higher stages of development depend; 



