The four sentences (labelled 1, 2, 3, 4) below, when properly sequenced would yield a coherent paragraph. Decide on the proper sequencing of the order of the sentences and key in the sequence of the four numbers as your answer:

1. Complex computational elements of the CNS are organized according to a “nested” hierarchic criterion; the organization is not permanent and can change dynamically from moment to moment as they carry out a computational task.
2. Echolocation in bats exemplifies adaptation produced by natural selection; a function not produced by natural selection for its current use is exaptation --feathers might have originally arisen in the context of selection for insulation.
3. From a structural standpoint, consistent with exaptation, the living organism is organized as a complex of “Russian Matryoshka Dolls” -- smaller structures are contained within larger ones in multiple layers.
4. The exaptation concept, and the Russian-doll organization concept of living beings deduced from studies on evolution of the various apparatuses in mammals, can be applied for the most complex human organ: the central nervous system (CNS).

The correct sequence is 2431.

Here is the step-by-step logical breakdown of the paragraph structure:

Step 1: Identifying the opening sentence
Sentence 2 introduces the fundamental biological concepts of adaptation (using echolocation in bats as an example) and exaptation (using feathers as an example). Because it introduces these key concepts without referring to prior context, it serves as the logical starting point of the paragraph.

Step 2: Connecting the general concept to the target system
Sentence 4 builds directly on the definition established in Sentence 2. It introduces how "the exaptation concept" (from Sentence 2) and "the Russian-doll organization concept" can be applied specifically to the human central nervous system (CNS). This acts as a bridge from general evolutionary theory to human neurology.

Step 3: Explaining the structural metaphor
Sentence 3 details the structural aspect of the "Russian-doll organization" mentioned in Sentence 4. It explains that, consistent with exaptation, living organisms are organized like "Russian Matryoshka Dolls," where smaller structures are nested within larger ones in multiple layers.

Step 4: Concluding with the dynamic nature of the target system
Sentence 1 completes the paragraph by detailing how this nested hierarchic criterion works specifically within the CNS. It explains that the computational elements of the CNS are organized according to this nested structure, but notes that this organization is dynamic and can change moment to moment during tasks.

By arranging the sentences in the sequence 2-4-3-1, the paragraph flows coherently from defining a general evolutionary principle, proposing its application to the human brain, describing the structural framework, and finally explaining how the brain dynamically utilizes this structure.