64 THE LIVING WORLD. 



organisms occupying the lowest position in the scale 

 of nature, and to find all animals and plants begin- 

 ning their embryology as single cells, were coinci- 

 dences of remarkable interest. There can be only 

 one interpretation of this. Since embryology is an 

 epitomized account of past history, the fact that all 

 animals and plants begin life as single cells, of course 

 must mean that, if we could follow back the history 

 of animals and plants, we should in each case finally 

 come to some unicellular organism. A unicellular 

 organism was therefore a common starting-point of 

 all animals and plants, and the Protozoa and Proto- 

 phyta of to-day are of interest as being close repre- 

 sentatives of the earliest organisms of which we have 

 any suggestion in our recorded history. 



This conclusion of a unicellular starting-point of 

 all animals and plants is a significant result of bio- 

 logical study. At the same time, as was seen in the 

 last chapter, more recent and exhaustive study of 

 cells is beginning to show that the simple cell is not 

 itself the unit of life, but is a complex body. Pro- 

 cesses are found to take place inside the cell which 

 indicate that we are still far from the unit of life. It 

 is seen that of the whole cell the nucleus is really the 

 essential part, and that it regulates the activities of 

 the rest (Fig. 2). The nucleus itself is moreover a 

 complex. No fewer than five different chemical 

 compounds have been found to exist in it. In struc- 

 ture also we find various fibres, liquids, and granules, 

 all of which seem to undergo definite cycles of change 

 in the activities of the cell. All of this indicates that 

 we are still far from the unit of life when we have 



