I.—PHYSIOLOGY. 213 
Conclusion. 
The term ‘ symbiosis’ denotes a condition of conjoint life existing 
between different organisms that in a varying degree are benefited by 
the partnership. The term ‘ symbiont,’ strictly speaking, applies 
equally to the partners; it has, however, come to be used also in a 
restricted sense as meaning the microscopic member or members of the 
partnership in contradistinction to the physically larger partners which 
are conveniently termed the ‘ hosts ’ in conformity with parasitological 
usage. 
The condition of life defined as symbiosis may be regarded as balancing 
between two extremes—complete immunity and deadly infective 
disease. A condition of perfect symbiosis or balance is realised with 
comparative rarity because of the many difficulties of its establishment 
in organisms that are either capable of Hving independently or are 
incapable of resisting the invasion of organisms imperfectly adapted to 
communal life. In these respects the conclusions of Bernard and 
Magrou in relation to plants apply equally to animals. It is difficult 
to imagine that symbiosis originated otherwise than through a pre- 
liminary stage of parasitism on the part of one or other of the associated 
organisms, the conflict, between them in the course of time ending in 
mutual adaptation. It is, indeed, probable that some supposed sym- 
bionts may prove to be parasites on further investigation. 
In perfect symbiosis the associated organisms are completely 
adapted to a life in common. In parasitism the degree of adaptation 
varies greatly; it may approach symbiotic conditions on the one hand, 
or range to vanishing point on the other by leading to the death of the 
organism that is invaded by a highly pathogenic animal or vegetable 
disease agent. There is no definite boundary between symbiosis and 
parasitism. The factors governing immunity from symbionts or para- 
sites are essentially the same. 
No final conclusions can as yet be reached regarding the function 
of symbionts in many invertebrate animals owing to our ignorance of 
the physiological processes in the associated organisms. The investiga- 
tion of these problems is one fraught with difficulties which we must 
hope will be surmounted. 
New knowledge is continually being acquired, and a glance into 
new and even recent publications shows that symbionts have been 
repeatedly seen and interpreted as mitochondria or chromidia. Thus 
in Aphis the long-known pseudovitellus has been shown to contain 
symbiotic yeasts by Pierantoni and Sule, independently and almost 
simultaneously (1910); Buchner (1914) has demonstrated symbiotic 
luminiscent fungi in the previously well-studied pyrosomes, besides 
identifying (1921) as bacterial symbionts the mitochondria found by 
Strindberg (1913) in his work on the embryology of ants. The increas- 
ing number of infective diseases of animals and plants, moreover, which 
have been traced, especially of recent years, to apparently ultramicro- 
scopic organisms cannot but suggest that there may exist ultra- 
microscopic symbionts. 
From the foregoing summary of what is known to-day of symbiosis 
