THE DISCOVERER 69 



method, with its demonstration of fundamental rela- 

 tions between living things. Schleiden discovered 

 that the cell is the unit of plant-life ; and Schwann 

 proved that the same is true of animals. Harvey's 

 formula of development, " All life comes from an 

 egg " (omne vivum ex ovo), gave place to the doctrine 

 of omnis cellula e cellula. The lowest animals are one- 

 celled, or, sometimes, a loosely connected cluster of 

 cells ; all other animals are built up of a number of 

 cells, whence tissues and organs are developed. In 

 1844, five years after Schwann's demonstration, Von 

 Mohl showed that each cell contains a viscous, gran- 

 ular-looking, highly active substance, the result of a 

 very complex union of carbon (to which Haeckel as- 

 signs the chief activity), hydrogen, oxygen, and nitro- 

 gen. This substance is known as protoplasm, and is, 

 in Huxley's familiar phrase, " the physical basis of 

 life." Some years before this, Von Baer had observed 

 that the embryos of birds, dogs, fish, and other back- 

 boned animals, including man, are all alike during 

 their earlier stages. It is concerning Von Baer's 

 writings that Huxley said none had made so great an 

 impression on him down to the publication of the 

 Origin of Species 1 ; and it was in Von Baer's Law 

 of Development that Mr. Spencer found hints and 

 evidence supporting his own theory of advance 



1 1. 175, and cf. 163. 



