SYMBIONTICISM AND ORGANIC EVOLUTION 141 



chondria are or are not concerned with secretion. (See 

 discussion, p. 44.) So also, a number of investigators 

 have claimed that myofibrillae, the fibrillae of connective 

 tissue, pigment granules, and other cellular products, arise 

 from mitochondria, and still others claim that mitochondria 

 are not involved in these activities. The relation of mito- 

 chondria to plast formation, on the other hand, appears 

 to have been accepted by most biologists, even by those 

 who oppose the claims made for mitochondrial activity in 

 animal tissues. It is difficult to understand or appreciate 

 the basis for such variations in interpretation, even on the 

 assumed incompatible chemical constitution of mitochon- 

 dria, for obviously, if mitochondria can transform or dif- 

 ferentiate into plastids containing chlorophyl, there is no 

 reason for assuming that they are incapable of differentiat- 

 ing into bodies with diverse properties. If all mitochondria 

 are presumed to be aUke in chemical constitution, then 

 we should surely expect to find chlorophyl in all cells. On 

 the contrary, it is quite evident that all mitochondria are 

 not aUke. Recent studies have shown that they are not 

 all alike in a single cell. 



We have indicated the method by which one-celled 

 plants and animals might have originated from the bacteria. 

 Obviously, the different species of protozoa and algae arose 

 from symbiotic combinations of different strains of bacteria 

 with the primordial cells. The algae deviated from the 

 protozoa in having prototactic properties that attracted 

 blue-green algal symbionts. One protozoan, at least, 

 appears to possess prototactic properties that are some- 

 what akin to the algae. This form is Euglena viridis which 

 contains chloroplasts. Bacterial sjmibionts (mitochondria) 

 have also been demonstrated in the algae. There are many 

 species of algae, and we are justified in assuming that the 

 different species owe their differences to the bacterial sym- 

 bionts (mitochondria) which they harbor. So also, in the 



