W. D. Lang — Bernard's Biological Theories of Fossils. 553 



degree than a cell, from which higher metazoa were built; and he 

 thinks he lias found this in the 'gastrsea' — the simplest expression 

 of the morphic principle of the Coeleuterate ; and above the gastraea 

 two more units of progressively higher order. In each case more 

 complex organisms are formed by aggregations of similar units, and 

 one specially efficient form of grouping constitutes a new unit from 

 which the process may begin again. 



Hitherto organic evolution has been looked upon as an advance 

 from the simple to the complex, from the homogeneous to the 

 heterogeneous, and the advance along any one line as straightforward, 

 though the steps may be more or less gradual or sudden. But, in 

 the light of different orders of units, organic evolution is seen to 

 proceed in periods, and periodic or rhythmic evolution is the theme 

 of the second part of Bernard's book. 



How many pre-cellular periods have occurred we do not know, but 

 we know one — that of the protomitomic network seen in certain 

 organisms (e.g. microbes and other non-nucleated protoplasmic 

 bodies), and we may reasonably expect that earlier periods behaved 

 similarly to this and to all subsequent periods. Now one funda- 

 mental character common to the units of all periods is an alternation 

 of growth and fission ; the unit cannot surpass a certain maximum 

 size, and when this is reached it divides by fission (secondarily 

 modified into budding). The simplest conceivable unit is a molecule 

 of protoplasm. If such existed as a period, we must allow that it 

 differed from every inorganic molecule by the possession of this 

 periodic rhythm of growth and fission as well as the two next- 

 mentioned characters. The first of these is the capacity of forming 

 colonies by means of incomplete fission. The colonies are formed in 

 great variety of manner, and one particularly efficient arrangement 

 forms a new unit. A third fundamental property of the units is 

 that of morphic response to the environment. That is, by exploring 

 new environments, their structure (often only after the lapse of 

 generations) becomes variously and suitably modified. Now the 

 process of organic evolution seems to have been as follows : — 

 A structural unit (of whatever order) multiplying by fission spreads 

 in all directions into new environments and becomes accordingly 

 variously modified, i.e. it passes from the simple to the complex, 

 from the homogeneous to the heterogeneous. Simultaneously, by 

 incomplete fission it forms colonies — homogeneous aggregates, whose 

 units, however, are variously arranged ; and one especially efficient 

 arrangement becomes a new unit. The process is then repeated as 

 a second period, and so on. 



We have spoken of a hypothetical molecular period. Other periods 

 are passed through until a pre-cellular period of which we have 

 knowledge is attained. This has the form of a chromidium with 

 radiating linin filaments. A colony of such units is the protomitomic 

 network. One particulai'ly effective arrangement of this is the cell, 

 formed by a massing of all the chromidial nodes into a central 

 nucleus. The cell is the new unit of a new (cellular) period. As 

 a unit it has advanced from the homogeneous simple form to the 

 heterogeneous complex structures such as Faramoecium and the 



