THE BUILDING-STONES OF THE ORGANIC WORLD 61 



that they regarded it as sacrilege to peer deeper into the ' secrets 

 of Nature which, according to Divine will, were to be for ever 

 hidden from human eyes.' It was only very gradually that 

 objective investigation availed itself in full of the new achieve- 

 ments of technical skill, and to-day we owe almost all the vast 

 progress of modern science to the insight into the interior of 

 organic bodies granted by the microscope. 



When we observe the world of animal and plant organisms 

 we find in the structure of their bodies a fundamental and 

 complete likeness : they are composed of an immense number 

 of minute homogeneous building-stones which we are accus- 

 tomed to call cells. Whichever organ we may examine with 

 the microscope, we shall always find that it is composed of 

 single cells ; and when we descend into the world of invisible 

 organisms, known to most men not even by name, and observe 

 the simplest and lowest animals and plants, we shall find that 

 they are nothing else but free-living single cells. 



But the most important discovery was that even the 

 highest and most complex organism, composed of many million 

 cells, whether man, animal, or plant, consists at the starting- 

 point of its individual existence of but a single cell the ovum, 

 or, more accurately, the impregnated cell, for, as a rule, it is 

 necessary that two cells, the female ovum and the male sperma- 

 tozoon, become united in order to render the production of a 

 new organism possible. Through many divisions of the egg- 

 cell, the differentiation of the individual parts, according to the 

 duties which they will later be called upon to perform, and the 

 fusion of homogeneous cells, tissues are formed ; these unite 

 to form the various organs which constitute the fully developed 

 organism, whether animal or plant. 



We may conveniently point here to an error committed by 

 many persons. If they are asked to state the difference between 

 an animal and a plant they will not hesitate to mention a 

 large number of seeming distinctions. They do so because most 

 persons think in such a question only of the types of both natural 

 kingdoms known to them, that is, the most highly organized and 

 most complex. The distinctive features between a pear-tree or 

 a poplar, a fern or a blade of grass on one side, and a lion, horse, 

 worm, butterfly or crab on the other are, of course, so obvious and 



