Cannot understand command substitution in Fish shell

Question

In sh:

~$ `echo ls`
bin/  Desktop/

But in fish:

fish: Illegal command name “(echo ls)”
~% (echo ls)

(Note

Answer 1

It has to do with the order of expansions.

From help expand-command-substitution in fish:

When combining multiple parameter expansions, expansions are performed in the following order:

 * Command substitutions
 * Variable expansions
 * Bracket expansion
 * Pid expansion
 * Wildcard expansion

Expansions are performed from right to left, nested bracket expansions are performed from the inside and out.

From man bash:

The order of expansions is: brace expansion, tilde expansion, parameter, variable and arithmetic expansion and command substitution (done in a left-to-right fashion), word splitting, and pathname expansion.