What is the relation between $3$-Ethylpentane and $3$-Methylhexane?

($\mathrm{I}$) ${\mathrm{CH}}_3-{\mathrm{CH}}_2-\mathrm{NH}-\mathrm{CHO}$

($\mathrm{II}$) ${\mathrm{CH}}_3-\underset{\stackrel{|}{{\mathrm{NH}}_2}}{\mathrm{CH}}-\mathrm{CHO}$

Which type of isomerism is observed between $\mathrm{I}$ and $\mathrm{II}$?